The word 'Anna' in PuruSa Suktam

Late yes, but not forgotten.

The word, as pointed out by Flute occurs in the phrase 'yadannenAtirohati' (yat + annena + atirohati).
It means "That which (yat) ascends (atirohati) by anna (annena--thRtIya vibhakti)".

Taken from Late Dr.S.K.Ramachandra Rao's 'PuruSa sukta':
It is usual to derive the word from 'ad' which has the sense of eating. Anna is what is eaten by living beings. It is also what eats or consumes them... i.e., karma phala. Therefore, anna means the material objects taken in by living beings to survive, and it means the karma proclivities acquired by living beings while procuring food and consuming it. All living beings are born of such food and survive on food.

There is another interpretation which takes anna in almost the same sense: the word refers to the world of actions and the results appropriate to them, comprehending this physical world that is unreal, and also the heavenly realms which are reached by ritualistic and religious actions, and which are also unreal. But the expression 'atirohati' is here taken in altogether a different sense: 'a-tirohati', 'does not dissappear or vanish'. The import is that the amrtatva (in the previous phrase) of which, the purusa is the lord, is not obscured or obliterated by the phenomenal world of actions and reactions. It is the real, and cannot be undone by appearances.

However, the nirukta nighantu asks "what is anna?" and answers: 'anna is so called because living beings incline or bend in its direction, or because it is consumed'. The word 'annam' derived from 'ad' as seen earlier, can also be derived from 'an', 'to breathe'. The nighantu gives:
1. andha: that by which creatures live,
2. brahma: that which makes the creatures grow, and which grows because of the creatures, and
3. varcha: that which gives lustre to the body
as synonymous with anna.
anna, derived from the root which has the meaning of movement or going, has by the same fact the meaning of knowledge. anna, therefore would mean knowledge. According to Siddhanta-Kaumudi, anna means the Sun. Thus, definition given in nairukta nighantu would apply equally justifiable to all these words: food, prAna, brahma, jnAna, surya.

Mundaka upanishad declares:: by tapas Brahma emanated; and from Brahma came forth anna, and from anna emerged prAna, as also manas, satya, lokAh, and amRta in actions.

The expression 'atirohati', which literally means mounts up, ascends, climbs suggests an upward movement, a transcendence, growth into a higher state or level of being. There are several mantras in RigVeda (RV) where the same expression is employed mostly with reference to Surya.
Now, Agni (the devatA of the earth region) is frequently mentioned in connection with anna in RV and likewise, Indra (the devatA of the mid-region) is associated with anna almost as often. Thus the expression 'annena atirohati' becomes meaningful, when viewed against the background of surya's mission of rising from the earth region and mid-region into the celestial region, assisted by Agni and Indra.

There is a lot more written in this connection in the book.