Rookie shooter (4)
I believe the answer is:
colt
This clue is a double definition.
'rookie' is the first definition.
(thesaurus)
'shooter' is the second definition.
(I've seen this before)
(Other definitions for colt that I've seen before include "shooter" , "Youngster" , "Male pony" , "Young Arab maybe" , "Young horse - gun" .)