Thus, it's the heart of the musical (3)
I believe the answer is:
sic
'thus' is the definition.
('thus' in Latin)
'the heart of the musical' is the wordplay.
'the heart of' says the answer is hidden in the clue.
'SIC' is hidden within 'musical'.
'it's' is the link.
(Other definitions for sic that I've seen before include "Like" , "'Thus, in printing (3)'" , "Word to call attention to a quoted mistake" , "Thus in Corsica" , "poorly spelt if requiring this" .)