Employer you serve 'heartily' (4)
I believe the answer is:
user
'employer' is the definition.
(someone who employs or uses something)
'you serve heartily' is the wordplay.
'heartily' indicates the answer is hidden within the clue (letters at the heart of the clue).
'USER' is hidden in the letters of 'you serve'.
(Other definitions for user that I've seen before include "Right long-established" , "one has to employ" , "Substance taker" , "One taking advantage" , "One taking drugs" .)