Dear partner previously deep in thought (9)
I believe the answer is:
expensive
'dear' is the definition.
(similar in meaning)
'partner previously deep in thought' is the wordplay.
'partner previously' becomes 'ex' (ex-partner).
'deep in thought' becomes 'pensive' (I've seen this before).
'ex'+'pensive'='EXPENSIVE'
(Other definitions for expensive that I've seen before include "High in price" , "Highly-priced" , "Very costly or dear" , "priceless" , "Pricey" .)