Left part of a band one deserted (9)
I believe the answer is:
abandoned
'left' is the definition.
(abandoning is a kind of leaving)
'a band one deserted' is the wordplay.
'deserted' becomes 'd'.
'a'+'band'+'one'+'d'='ABANDONED'
'part of' is the link.
(Other definitions for abandoned that I've seen before include "without any reservations at all" , "Quit" , "Unrestrained" , "Left in the lurch, forsaken" , "Deserted - shameless" .)