One of pack in charge holding ace of hearts (7)
I believe the answer is:
cardiac
'of hearts' is the definition.
(relating to the heart)
'one of pack in charge holding ace' is the wordplay.
'one of pack' becomes 'card' (one from a pack of cards).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'holding' is an insertion indicator.
'ace' becomes 'a'.
'ic' enclosing 'a' is 'iac'.
'card'+'iac'='CARDIAC'
(Other definitions for cardiac that I've seen before include "Sort of surgery" , "one with a heart disorder" , "of hearts" , "Pertaining to the heart" , "Relating to the heart" .)