Pressure on unhealthy to get some medicine (4)
I believe the answer is:
pill
'some medicine' is the definition.
(pill is a kind of medicine)
'pressure on unhealthy' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' means one lot of letters go next to another.
'unhealthy' becomes 'ill' ('ill' is associated in meaning with 'unhealthy').
'p'+'ill'='PILL'
'to get' is the link.
(Other definitions for pill that I've seen before include "Dose of medicine" , "Medicinal tablet" , "Pellet of medicine" , "Medical tablet" , "Form of medicine" .)