Bean curd content of uneaten sandwiches? (4)
I believe the answer is:
tofu
'bean curd' is the definition.
(synonyms)
'content of uneaten sandwiches?' is the wordplay.
'sandwiches?' indicates the answer is hidden within the clue (I've seen 'sandwiches' mean this (the answer is sandwiched in by the clue)).
'TOFU' can be found hidden inside 'content of uneaten'.
(Other definitions for tofu that I've seen before include "'Soft white food, made of soy milk (4)'" , "Unfermented soya bean curd" , "Substitute for meat" , "Soya bean dish" , "food rich in protein" .)