Cast fly, initially, to get fish (5)
I believe the answer is:
fling
'cast' is the definition.
(both can mean to throw)
'fly initially to get fish' is the wordplay.
'initially' suggests taking the first letters.
'to get' is a charade indicator (letters next to each other).
'fish' becomes 'ling' (ling is a kind of fish).
The first letter of 'fly' is 'f'.
'f'+'ling'='FLING'
(Other definitions for fling that I've seen before include "Brief relationship" , "Toss" , "Spell of indulgence" , "Throw angrily in spell of indulgent pleasure" , "Throw with force" .)