Pass trainer's first young player (4)
I believe the answer is:
colt
'young player' is the definition.
(young member of a sports team)
'pass trainer's first' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'trainer's first' becomes 't' (cst letter of 'trainer').
'col'+'t'='COLT'
(Other definitions for colt that I've seen before include "Male foal" , "Early American revolver" , "shooter" , "Young animal" , "Young stallion" .)