Valuable possession (5)
'valuable possession' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for asset that I've seen before include "Useful item" , "Something well worth having" , "virtue" , "A thing of value" , "valuable sort" .)
(Other definitions for treasure that I've seen before include "A quantity of valuable objects" , "... story by R.L. Stevenson" , "Accumulated wealth, might be buried" , "Hold dear, prize" , "Gold on this island" .)