Insufficient money for a rich biscuit (10)
I believe the answer is:
shortbread
'a rich biscuit' is the definition.
(I've seen this before)
'insufficient money' is the wordplay.
'insufficient' becomes 'short' ('short' can be similar in meaning to 'insufficient').
'money' becomes 'bread' (bread is a kind of money).
'short'+'bread'='SHORTBREAD'
'for' acts as a link.
(Other definitions for shortbread that I've seen before include "Crumbly biscuit rich in butter" , "Brittle buttery cake" , "sweet but easily crumbling" , "Biscuit with plenty of butter" , "biscuits" .)