Regular heartbeat at the end in race (5)
I believe the answer is:
event
'race' is the definition.
(thesaurus)
'regular heartbeat at the end' is the wordplay.
'regular' becomes 'even' ('even' can be similar in meaning to 'regular').
'at the end' says to take the final letters.
The last letter of 'heartbeat' is 't'.
'even'+'t'='EVENT'
'in' is the link.
(Other definitions for event that I've seen before include "game, perhaps" , "Happening, incident" , "Affair" , "ride competitively" , "Something happening" .)