Instrument cheers knight outside (5)
I believe the answer is:
sitar
'instrument' is the definition.
(sitar is a kind of instrument)
'cheers knight outside' is the wordplay.
'cheers' becomes 'ta' (I've seen this in other clues).
'knight' becomes 'sir' (I've seen this before).
'outside' is an insertion indicator.
'ta' going inside 'sir' is 'SITAR'.
(Other definitions for sitar that I've seen before include "Plucked instrument" , "Lasting from morning to night" , "instrument out east" , "music-maker" , "Long necked and often plucked" .)