Thoughtful, dear former lover departing (7)
I believe the answer is:
pensive
'thoughtful' is the definition.
(similar in meaning)
'dear former lover departing' is the wordplay.
'dear' becomes 'expensive' (similar in meaning).
'former lover' becomes 'ex'.
'departing' is a deletion indicator.
'expensive' with 'ex' taken away is 'PENSIVE'.
(Other definitions for pensive that I've seen before include "Meditative" , "Preoccupied" , "Engaged in deep thought" , "Deeply thoughtful" , "musing" .)