Friend with a false heart on the quiet (3)
I believe the answer is:
pal
'friend' is the definition.
(pal can mean a friend)
'a false heart on the quiet' is the wordplay.
'heart' means to look at the middle letters.
'on' says to put letters next to each other.
'the quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
The centre of 'false' is 'l'.
'a'+'l'='al'
'al' put after 'p' is 'PAL'.
'with' acts as a link.
(Other definitions for pal that I've seen before include "Friend (colloq.)" , "China" , "Bud" , "Close friend" , "Crony" .)