Dear old partner, deeply thoughtful (9)
I believe the answer is:
expensive
'dear' is the definition.
(similar in meaning)
'old partner deeply thoughtful' is the wordplay.
'old partner' becomes 'ex' (an ex-partner).
'deeply thoughtful' becomes 'pensive' (I've seen this before).
'ex'+'pensive'='EXPENSIVE'
(Other definitions for expensive that I've seen before include "High in price" , "priceless" , "Highly-priced" , "Costing a lot of money" , "Charging a lot" .)