Initially grill a river fish (3)
I believe the answer is:
gar
'fish' is the definition.
(gar is a kind of fish)
'initially grill a river' is the wordplay.
'initially' indicates taking the first letters.
'river' becomes 'r'.
The initial letter of 'grill' is 'g'.
'g'+'a'+'r'='GAR'
(Other definitions for gar that I've seen before include "Creature" , "A fish; cloth scrap (reversed)" , "Type of fish" , "Pike-like fish" , "swimmer" .)