This writer left poem on musical instrument (8)
I believe the answer is:
melodeon
'musical instrument' is the definition.
(I've seen this before)
'this writer left poem on' is the wordplay.
'this writer' becomes 'me' (the person who is writing these clues).
'left' becomes 'l' (common abbreviation).
'poem' becomes 'ode' (ode is a kind of poem**).
'me'+'l'+'ode'+'on'='MELODEON'
(Other definitions for melodeon that I've seen before include "Small squeezebox" , "A small accordion" , "music-maker" , "Kind of small reed organ" , "A kind of accordion" .)