Dear old flame is thinking about.. (9)
I believe the answer is:
expensive
'dear' is the definition.
(similar in meaning)
'old flame is thinking about' is the wordplay.
'old flame' becomes 'ex' (a former partner).
'is thinking about' becomes 'pensive' (I can't explain this - if you can you should give a lot more credence to this answer).
'ex'+'pensive'='EXPENSIVE'
Can you help me to learn more?
(Other definitions for expensive that I've seen before include "not going for a song" , "Pricey" , "Highly-priced" , "Charging a lot" , "High in price" .)