Left one with the part that is supple (9)
I believe the answer is:
lithesome
'is supple' is the definition.
('lithesome' can be a synonym of 'supple')
'left one with the part' is the wordplay.
'left one' becomes 'li' (left abbreviation plus 1).
'with' is a charade indicator (letters next to each other).
'part' becomes 'some' (I've seen this before).
'li'+'the'+'some'='LITHESOME'
'that' acts as a link.
(Other definitions for lithesome that I've seen before include "flexibility" , "slim and supple" , "Flexible" , "Graceful" , "Willowy" .)