Fish, we hear, that's looked at (4)
I believe the answer is:
eyed
'looked at' is the definition.
(I've seen this in another clue)
'fish we hear' is the wordplay.
'fish' becomes 'ide' (type of fish).
'we hear' shows a homophone (sound like).
'ide' is a homophone of 'EYED'.
'that's' is the link.
(Other definitions for eyed that I've seen before include "examined" , "Watched with interest" , "Watched warily" , "Saw" , "Spotted" .)