Watch the heart (6)
I believe the answer is:
ticker
I believe this is a double definition.
'watch' is the first definition.
('ticker' can be a synonym of 'watch')
'the heart' is the second definition.
('ticker' can be a synonym of 'heart')
(Other definitions for ticker that I've seen before include "Slang heart or old spring watch" , "Heart (sl.)" , "Old clock or slang heart (6)" , "teacher giving correct marks" , "- - -tape machine" .)