Relate to a forward in agony (9)
I believe the answer is:
Here is my best explanation:
'relate' is the definition.
'a forward in agony' is the subsidiary indication.
'forward' becomes 'pert' (I've seen this in other clues).
'in' indicates putting letters inside.
'agony' becomes 'pain' (I've seen this before).
'pert' going within 'pain' is 'ppertain'.
'to' is the link.
(Other definitions for appertain that I've seen before include "Bear on", "Be relevant to", "Concern", "Belong, relate (to)" and "Relate (to)".)