Unwilling promise by left (5)
I believe the answer is:
loath
'unwilling' is the definition.
(similar in meaning)
'promise by left' is the wordplay.
'promise' becomes 'oath' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'oath' after 'l' is 'LOATH'.
(Other definitions for loath that I've seen before include "Dislike intensely" , "Reluctant or averse" , "Hateful" , "Unwilling" .)