Dear former lover deep in thought (9)
I believe the answer is:
expensive
'dear' is the definition.
('expensive' can be similar in meaning to 'dear')
'former lover deep in thought' is the wordplay.
'former lover' becomes 'ex'.
'deep in thought' becomes 'pensive' (I've seen this before).
'ex'+'pensive'='EXPENSIVE'
(Other definitions for expensive that I've seen before include "not going for a song" , "Highly-priced" , "Pricey" , "High in price" , "priceless" .)