Small, small number for sugar (7)
I believe the answer is:
sweeten
'for sugar' is the definition.
(I've seen this before)
'small small number' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'small' becomes 'wee' (Scottish term for small).
'number' becomes 'ten' (ten is a kind of whole number).
's'+'wee'+'ten'='SWEETEN'
(Other definitions for sweeten that I've seen before include "placate" , "We teens! (anag)" , "Add, eg, sugar to" , "Add sugar etc" , "pacify!" .)