Box in occasional Scottish players (9)
I believe the answer is:
orchestra
'players' is the definition.
(group playing music)
'box in occasional scottish' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'in' indicates putting letters inside.
'occasional scottish' becomes 'orra' (Scottish term meaning 'occasional').
'chest' placed into 'orra' is 'ORCHESTRA'.
(Other definitions for orchestra that I've seen before include "Group of musicians found on cart-horse" , "Musical ensemble" , "Instrumental group" , "Musicians, maybe in theatre" , "alternative to the organ" .)