Thoughtful to support former partner, and costly! (9)
I believe the answer is:
expensive
'costly' is the definition.
('expensive' can be similar in meaning to 'costly')
'thoughtful to support former partner' is the wordplay.
'thoughtful' becomes 'pensive' ('pensive' can be similar in meaning to 'thoughtful').
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'former partner' becomes 'ex'.
'pensive' put after 'ex' is 'EXPENSIVE'.
'and' acts as a link.
(Other definitions for expensive that I've seen before include "Pricey" , "Very costly or dear" , "not going for a song" , "Highly-priced" , "Charging a lot" .)