Although poor he's rich in treasure (7)
I believe the answer is:
cherish
'treasure' is the definition.
(I know that treasure can be written as cherish)
'although poor he's rich in' is the wordplay.
'although' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'poor' indicates anagramming the letters.
'in' indicates putting letters inside.
'he'+'rich'='herich'
'herich' with letters rearranged gives 'cherih'.
's' inserted inside 'cherih' is 'CHERISH'.
Can you help me to learn more?
(Other definitions for cherish that I've seen before include "Love" , "Protect and care for lovingly" , "Protect and treat with affection" , "Hold dear, keep in one's heart" , "Hold dear, treasure" .)