Qualities oddly forgotten entering a boring affair (8)
I believe the answer is:
adultery
'affair' is the definition.
(thesaurus)
'qualities oddly forgotten entering a boring' is the wordplay.
'oddly forgotten' means one should take alternating letters (odd-numbered letters get removed).
'entering' means one lot of letters goes inside another.
'boring' becomes 'dry'.
The alternating letters of 'qualities' are 'ulte'.
'a'+'dry'='adry'
'ulte' put within 'adry' is 'ADULTERY'.
(Other definitions for adultery that I've seen before include "Extramarital affair" , "Marital infidelity - try a duel (anag)" , "Act of infidelity by husband or wife" , "form of betrayal" , "Sexual unfaithfulness to spouse" .)