Dear old lover deep in thought (9)
I believe the answer is:
expensive
'dear' is the definition.
(similar in meaning)
'old lover deep in thought' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'deep in thought' becomes 'pensive' (I've seen this before).
'ex'+'pensive'='EXPENSIVE'
(Other definitions for expensive that I've seen before include "priceless" , "Highly-priced" , "not going for a song" , "Charging a lot" , "Very costly or dear" .)