Kid upset by fish (3)
I believe the answer is:
gar
'fish' is the definition.
(gar is a kind of fish)
'kid upset' is the wordplay.
'kid' becomes 'rag' (I've seen this in other clues).
'upset' shows that the letters should be reversed in order.
'rag' in reverse letter order is 'GAR'.
'by' is the link.
(Other definitions for gar that I've seen before include "swimmer" , "Creature" , "Pike-like fish" , "Type of fish" , "A fish; cloth scrap (reversed)" .)