Important food not fresh, saving little money (6)
I believe the answer is:
staple
'important food' is the definition.
(as in staple of one's diet)
'not fresh saving little money' is the wordplay.
'not fresh' becomes 'stale' ('stale' is the opposite to 'fresh').
'saving' indicates putting letters inside.
'little money' becomes 'p' (pence).
'stale' going around 'p' is 'STAPLE'.
(Other definitions for staple that I've seen before include "Paper pin - basic" , "Paper fastening" , "Pleats in a paper pin" , "Fastening device" , "A wire paper fastener for plates" .)