Officer's sound heart (7)
'officer's' is the definition.
(colonel is a kind of officer)
'sound heart' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'heart' becomes 'kernel' ('kernel' can be a synonym of 'heart').
'kernel' sounds like 'COLONEL'.
(Other definitions for colonel that I've seen before include "Senior army officer" , "Army rank" , "Comissioned military officer" , "Military rank" , "Commissioned officer" .)
'heart' is the definition.
('kernel' can be a synonym of 'heart')
'officer's sound' is the wordplay.
'officer' becomes 'colonel' (colonel is a kind of officer).
'sound' shows a homophone (sound like).
'colonel' sounds like 'KERNEL'.
(Other definitions for kernel that I've seen before include "Heart and soul" , "Inner part of nut" , "Edible or central part of nut" , "Inner seed of nut" , "soft heart" .)