Sheep trapped in sewer (3)
I believe the answer is:
ewe
'sheep' is the definition.
(a ewe is a female sheep)
'trapped in sewer' is the wordplay.
'trapped in' says the answer is hidden in the clue.
'EWE' is hidden within 'sewer'.
(Other definitions for ewe that I've seen before include "Scottish loch" , "Female animal" , "A palindromic sheep" , "Wool supplier" , "Female in a fleece?" .)