Carefully looked at fish, it's said (4)
I believe the answer is:
eyed
'carefully looked at' is the definition.
(to eye something is to study it carefully)
'fish it's said' is the wordplay.
'fish' becomes 'ide' (type of fish).
'it's said' shows a homophone (sound like).
'ide' is a homophone of 'EYED'.
(Other definitions for eyed that I've seen before include "Closely observed" , "Saw" , "Watched with interest" , "Spotted" , "Watched, looked at" .)