Advent of a hard-hearted competitor (7)
I believe the answer is:
arrival
'advent' is the definition.
(thesaurus)
'a hard-hearted competitor' is the wordplay.
'hearted' means to look at the middle letters.
'competitor' becomes 'rival' ('rival' can be a synonym of 'competitor').
The middle of 'hard' is 'ar'.
'ar'+'rival'='ARRIVAL'
'of' is the link.
(Other definitions for arrival that I've seen before include "Coming - comer" , "Advent" , "Appearance (at a destination)" , "The reaching of one's destination" , "Act of coming" .)