Not having enough money for a biscuit (10)
I believe the answer is:
shortbread
'a biscuit' is the definition.
(shortbread is a kind of biscuit)
'not having enough money' is the wordplay.
'not having enough' becomes 'short' (as in being short of a required amount).
'money' becomes 'bread' (bread is slang for money).
'short'+'bread'='SHORTBREAD'
'for' is the link.
(Other definitions for shortbread that I've seen before include "biscuits" , "Biscuit with plenty of butter" , "Brittle buttery cake" , "Crumbly biscuit rich in butter" , "sweet but easily crumbling" .)