Animal trapped in the well (3)
I believe the answer is:
ewe
'animal' is the definition.
(female sheep)
'trapped in the well' is the wordplay.
'trapped in' says the answer is hidden in the clue.
'EWE' can be found hidden inside 'the well'.
(Other definitions for ewe that I've seen before include "Sheep found in sewer" , "Wool producer" , "Wool supplier" , "creature" , "Lamb supplier" .)