In jail, guard picked up something to eat (4)
I believe the answer is:
ugli
'something to eat' is the definition.
(ugli is a kind of food)
'in jail guard picked up' is the wordplay.
'in' indicates a hidden word.
'picked up' is a reversal indicator.
'ilgu' is hidden in the letters of 'jail guard'.
'ilgu' backwards is 'UGLI'.
(Other definitions for ugli that I've seen before include "Hybrid fruit, sounds nasty" , "Sweet variety of tangelo from Jamaica" , "Tropical fruit" , "Citrus fruit cross (which sounds offensive!)" , "Grapefruit/tangerine hybrid" .)