Occurrence in flat to start (5)
I believe the answer is:
event
'occurrence' is the definition.
(I've seen this before)
'flat to start' is the wordplay.
'flat' becomes 'even' (level).
'start' means to remove the last letter (I've seen 'starts' mean this).
'to' with its last letter removed is 't'.
'even'+'t'='EVENT'
'in' is the link.
(Other definitions for event that I've seen before include "Occurence, happening" , "Fixture" , "Notable occasion" , "ride competitively" , "Occurren" .)