Keep food for sandwiches (5,3,6)
I believe the answer is:
bread and butter
'keep' is the definition.
(synonyms)
'food for sandwiches' is the wordplay.
I cannot quite understand how this works, but
'food' could be 'butter' (butter is a kind of food) and 'butter' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bread and butter that I've seen before include "Food" , "pudding" , "rather monotonous diet" , "Subsistence" , "earned?" .)