Confirm suitable returns in fish (6)
I believe the answer is:
ratify
'confirm' is the definition.
(thesaurus)
'suitable returns in fish' is the wordplay.
'suitable' becomes 'fit' ('fit' can be similar in meaning to 'suitable').
'returns' says the letters should be written in reverse.
'in' is an insertion indicator.
'fish' becomes 'ray' (ray is a kind of fish).
'fit' backwards is 'tif'.
'tif' put inside 'ray' is 'RATIFY'.
(Other definitions for ratify that I've seen before include "Approve and formally accept" , "Give validity to" , "Approve officially" , "Make (treaty) officially valid" , "Sanction" .)