The heart of a watch (6)
I believe the answer is:
ticker
I believe this clue is a double definition.
'the heart' is the first definition.
(informal term for the heart)
'a watch' is the second definition.
(I know that watch can be written as ticker)
'of' acts as a link.
(Other definitions for ticker that I've seen before include "Hunter, perhaps" , "timer" , "teacher giving correct marks" , "Old watch or informal heart" , "Heart like a clock" .)