Man left moccasin outside oval event (8)
I believe the answer is:
occasion
'event' is the definition.
(I know that occasion is a type of happening)
'man left moccasin outside oval' is the wordplay.
'man' becomes 'm' (M is an abbreviation for male or masculine).
'left' indicates named letters should be taken away.
'outside' is an insertion indicator.
'oval' becomes 'O'.
'moccasin' placed around 'o' is 'moccasion'.
'moccasion' with 'm' taken away is 'OCCASION'.
(Other definitions for occasion that I've seen before include "Provoke" , "Need" , "call" , "Time of (maybe special) event" , "excuse for > special ceremony" .)