Thoughtful to support former lover, dear (9)
I believe the answer is:
expensive
'dear' is the definition.
(similar in meaning)
'thoughtful to support former lover' is the wordplay.
'thoughtful' becomes 'pensive' (similar in meaning).
'to support' says to put letters next to each other (in a down clue, some letters hold up others).
'former lover' becomes 'ex' (an ex-lover).
'pensive' put after 'ex' is 'EXPENSIVE'.
(Other definitions for expensive that I've seen before include "Very costly or dear" , "priceless" , "Highly-priced" , "High in price" , "Charging a lot" .)