Instrument, one a bit rum when played (10)
I believe the answer is:
tambourine
'instrument' is the definition.
(tambourine is a kind of instrument)
'one a bit rum when played' is the wordplay.
'when played' is an anagram indicator (I've seen this in other clues).
'one'+'a'+'bit'+'rum'='oneabitrum'
'oneabitrum' anagrammed gives 'TAMBOURINE'.
(Other definitions for tambourine that I've seen before include "drum" , "Percussion instrument" , "Shaken instrument" , "One uses jingles" .)