Left rehab, and one deteriorated, to some extent (9)
I believe the answer is:
abandoned
'left' is the definition.
(abandoning is a kind of leaving)
'rehab and one deteriorated to some extent' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'ABANDONED' can be found hidden inside 'rehab and one deteriorated'.
(Other definitions for abandoned that I've seen before include "Left - dissolute" , "Dropped" , "Given up?" , "marooned" , "Quit" .)