Starter of grilled American river fish (3)
I believe the answer is:
gar
'fish' is the definition.
(gar is a kind of fish)
'starter of grilled american river' is the wordplay.
'starter of' suggests taking the first letters.
The first letters of 'grilled american river' is 'GAR'.
(Other definitions for gar that I've seen before include "Type of fish" , "Creature" , "Pike-like fish" , "swimmer" , "something fishy here" .)