Thoughtful after former partner is costing too much (9)
I believe the answer is:
expensive
'costing too much' is the definition.
'thoughtful after former partner' is the wordplay.
'thoughtful' becomes 'pensive' ('pensive' can be similar in meaning to 'thoughtful').
'after' is a charade indicator (letters next to each other).
'former partner' becomes 'ex'.
'pensive' after 'ex' is 'EXPENSIVE'.
'is' is the link.
(Other definitions for expensive that I've seen before include "priceless" , "Pricey" , "not going for a song" , "Highly-priced" , "Charging a lot" .)