First-class pain caused by treasure (5)
I believe the answer is:
cache
'treasure' is the definition.
(thesaurus)
'first-class pain' is the wordplay.
'first class' becomes 'c' (ast letter of 'class').
'pain' becomes 'ache' (ache is a kind of pain).
'c'+'ache'='CACHE'
'caused by' acts as a link.
(Other definitions for cache that I've seen before include "Concealed store" , "Secret store, hoard" , "Conceal" , "Nest egg" , "Collection of hidden things" .)