Everyone ate stolen sandwiches -- most smart! (7)
I believe the answer is:
neatest
'most smart' is the definition.
('neat' can be a synonym of 'smart')
'everyone ate stolen sandwiches' is the wordplay.
'sandwiches' indicates a hidden word (the answer is sandwiched in by the clue).
'NEATEST' is hidden within 'everyone ate stolen'.
(Other definitions for neatest that I've seen before include "Most smart" , "Most tidy" , "Seat ten (anag)" , "best-looking desk, say?" , "Most orderly" .)