My accepting a rotten rise leads to unhappiness (6)
I believe the answer is:
misery
'unhappiness' is the definition.
(I know that misery is a type of mournfulness)
'my accepting a rotten rise' is the wordplay.
'accepting' is an insertion indicator (inserted letters are accepted).
'a rotten' indicates an anagram.
'rise' is an anagram of 'iser'.
'my' enclosing 'iser' is 'MISERY'.
'leads to' is the link.
(Other definitions for misery that I've seen before include "Wretchedness, suffering" , "Mental distress or discomfort" , "Feeling of great unhappiness" , "''. . . . . . loves company''" , "Despair" .)