Medicine that makes you sick on the quiet? (4)
I believe the answer is:
pill
'medicine' is the definition.
(pill is a kind of medicine)
'sick on the quiet?' is the wordplay.
'sick' becomes 'ill' (synonyms).
'on' is a charade indicator (letters next to each other).
'the quiet?' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'ill' after 'p' is 'PILL'.
'that makes you' is the link.
(Other definitions for pill that I've seen before include "Medical tablet" , "Medication" , "Pellet of medicine" , "may be the cure" , "Lozenge" .)