Relate to a forward in agony (9)
I believe the answer is:
appertain
'relate' is the definition.
(thesaurus)
'a forward in agony' is the wordplay.
'forward' becomes 'pert' (I've seen this before).
'in' is an insertion indicator.
'agony' becomes 'pain' (I've seen this before).
'pert' placed within 'pain' is 'ppertain'.
'a'+'ppertain'='APPERTAIN'
'to' is the link.
(Other definitions for appertain that I've seen before include "Be relevant to" , "Bear on" , "Belong, relate (to)" , "Concern" .)