Rebuff concerned with heartbeat (7)
I believe the answer is:
repulse
'rebuff' is the definition.
(I know that rebuff can be written as repulse)
'concerned with heartbeat' is the wordplay.
'concerned with' becomes 're' (re can mean regarding or about).
'heartbeat' becomes 'pulse' (synonyms).
're'+'pulse'='REPULSE'
(Other definitions for repulse that I've seen before include "Drive back by force" , "Repel" , "Rebuff, drive back" , "kick in the teeth" , "Be disgusting - drive back" .)