Unhappy about page on the subject of food (6)
I believe the answer is:
spread
'food' is the definition.
(spread is a kind of food)
'unhappy about page on the subject of' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy'**).
'about' is an insertion indicator.
'page' becomes 'p'.
'on' means one lot of letters go next to another.
'the subject of' becomes 're' (I've seen this in other clues).
'p'+'re'='pre'
'sad' enclosing 'pre' is 'SPREAD'.
(Other definitions for spread that I've seen before include "Extend -- feast" , "Gradually reach a wider area" , "Broaden - drapes (anag)" , "Diffuse" , "Feature" .)