Observed freshwater fish, so to speak (4)
I believe the answer is:
eyed
'observed' is the definition.
(I've seen this before)
'freshwater fish so to speak' is the wordplay.
'freshwater fish' becomes 'ide'.
'so to speak' indicates a 'sounds like' (homophone) clue.
'ide' sounds like 'EYED'.
(Other definitions for eyed that I've seen before include "Looked at closely" , "Ogled" , "Watched warily" , "Vegetable" , "Stared at" .)