Heart in sound body (5)
I believe the answer is:
corps
'body' is the definition.
(I know that corps is a type of body)
'heart in sound' is the wordplay.
'heart' becomes 'core' (synonyms).
'in sound' shows a homophone (sound like).
'core' sounds like 'CORPS'.
(Other definitions for corps that I've seen before include "gang" , "Specialised mil. body" , "Group of soldiers" , "Army division" , "army band" .)