Advent calendar finally included by a competitor (7)
I believe the answer is:
arrival
'advent' is the definition.
(thesaurus)
'calendar finally included by a competitor' is the wordplay.
'finally' suggests the final letters.
'included by' is an insertion indicator.
'competitor' becomes 'rival' ('rival' can be a synonym of 'competitor'**).
The last letter of 'calendar' is 'r'.
'a'+'rival'='arival'
'r' placed within 'arival' is 'ARRIVAL'.
(Other definitions for arrival that I've seen before include "Act of coming" , "Appearance, entrance" , "Someone who's just come" , "The reaching of one's destination" , "Advent" .)