Incident in flat ending in arrest (5)
I believe the answer is:
event
'incident' is the definition.
(thesaurus)
'flat ending in arrest' is the wordplay.
'flat' becomes 'even' ('even' can be similar in meaning to 'flat').
'ending in' suggests the final letters.
The last letter of 'arrest' is 't'.
'even'+'t'='EVENT'
'in' is the link.
(Other definitions for event that I've seen before include "Occasion; sports contest" , "horse-riding competition" , "Happening, incident" , "Notable occasion" , "Occurence, occasion" .)