Achievement to hold small banquet (5)
I believe the answer is:
feast
'banquet' is the definition.
(I know that banquet can be written as feast)
'achievement to hold small' is the wordplay.
'achievement' becomes 'feat' (feat is a kind of achievement).
'to hold' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'feat' enclosing 's' is 'FEAST'.
(Other definitions for feast that I've seen before include "Repast" , "(Eat) a rich meal" , "'Enough is as good as a .....'" , "wine and dine" , "Belshazzar's party" .)