Sheep found in sewer (3)
I believe the answer is:
ewe
'sheep found in sewer' is the definition.
(I've seen this before)
'in sewer' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EWE' is hidden in the letters of 'sewer'.
(Other definitions for ewe that I've seen before include "Wool producer" , "Scottish loch" , "Ruminant" , "Lamb's mother" , "Farm animal" .)