Dear former lover, lost in thought (9)
I believe the answer is:
expensive
'dear' is the definition.
(similar in meaning)
'former lover lost in thought' is the wordplay.
'former lover' becomes 'ex' (ex-lover).
'lost in thought' becomes 'pensive' (I've seen this in another clue).
'ex'+'pensive'='EXPENSIVE'
(Other definitions for expensive that I've seen before include "Pricey" , "Costing a lot of money" , "not going for a song" , "Charging a lot" , "Highly-priced" .)