Large fish seen in Gibraltar pond (6)
I believe the answer is:
tarpon
'large fish' is the definition.
(I've seen this before)
'in gibraltar pond' is the wordplay.
'in' indicates a hidden word.
'TARPON' is hidden within 'gibraltar pond'.
'seen' is the link.
(Other definitions for tarpon that I've seen before include "Large silvery game fish of coastal waters off Florida" , "No part for this large fish" , "Great fish of Florida" .)