They happen to find an opening in two quarters (6)
I believe the answer is:
events
'they happen' is the definition.
(I've seen this before)
'an opening in two quarters' is the wordplay.
'two quarters' means two substitutions for 'quarter'.
'an opening' becomes 'vent' (vent is a kind of opening).
'in' means one lot of letters goes inside another.
'quarter' becomes 'e'.
'quarter' becomes 's'.
'e'+'s'='es'
'vent' put into 'es' is 'EVENTS'.
'to find' acts as a link.
(Other definitions for events that I've seen before include "Incidents, occurrences" , "Important happenings for Steven" , "Incidents, occasions" , "Noteworthy happenings" , "Occurrences, happenings" .)