Firm about the heart (4)
I believe the answer is:
core
'the heart' is the definition.
(synonyms)
'firm about' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'about' becomes 're' ('re' means regarding or about).
'co'+'re'='CORE'
(Other definitions for core that I've seen before include "The centre of an object" , "Crux" , "Heal" , "Central zone" , "The centre part, as of an apple" .)