Instrument — one held by leading player (5)
I believe the answer is:
sitar
'instrument' is the definition.
(sitar is a type of musical instrument)
'one held by leading player' is the wordplay.
'one' becomes 'i' (Roman numeral).
'held by' indicates putting letters inside.
'leading player' becomes 'star' (I've seen this before).
'i' put within 'star' is 'SITAR'.
(Other definitions for sitar that I've seen before include "Long-necked musical instrument of India" , "Guitar-like instrument" , "Hindu stringed instrument" , "Astir with Indian long-necked lute" , "Stringed instrument of the subcontinent" .)