Instrument this person left by theatre (8)
I believe the answer is:
melodeon
'instrument' is the definition.
(I've seen this before)
'this person left by theatre' is the wordplay.
'this person' becomes 'me' (the person writing).
'left by' becomes 'l' (I've seen this in other clues).
'theatre' becomes 'odeon' (in ancient Greece).
'me'+'l'+'odeon'='MELODEON'
(Other definitions for melodeon that I've seen before include "Kind of small reed organ" , "music-maker" , "Small squeezebox" , "A kind of accordion" , "Musical instrument" .)