Instruction for player — left winger, perhaps, entering pitch (10)
I believe the answer is:
allegretto
'instruction for player' is the definition.
(allegretto is a musical instruction)
'left winger perhaps entering pitch' is the wordplay.
'left' becomes 'l' (common abbreviation).
'winger perhaps' becomes 'egret' (type of bird).
'entering' means one lot of letters goes inside another.
'pitch' becomes 'alto' (musical pitch).
'l'+'egret'='legret'
'legret' placed into 'alto' is 'ALLEGRETTO'.
(Other definitions for allegretto that I've seen before include "quite quickly" , "somewhat quick" , "Moving fairly briskly (music)" , "at a fairly brisk rate" , "with reasonable rapidity" .)