The advent of a hard-hearted competitor (7)
I believe the answer is:
arrival
'the advent' is the definition.
(I've seen this before)
'hard-hearted competitor' is the wordplay.
'hearted' indicates the central letters.
'competitor' becomes 'rival' (synonyms).
The centre of 'hard' is 'ar'.
'ar'+'rival'='ARRIVAL'
'of a' acts as a link.
(Other definitions for arrival that I've seen before include "Attainment of destination" , "Appearance (at a destination)" , "The reaching of one's destination" , "Appearance (at a place)" , "Appearance, entrance" .)