Medicine makes top politician unwell (4)
I believe the answer is:
pill
'medicine' is the definition.
(pill is a kind of medicine)
'top politician unwell' is the wordplay.
'top' suggests taking the first letters.
'unwell' becomes 'ill' ('ill' can be similar in meaning to 'unwell').
The first letter of 'politician' is 'p'.
'p'+'ill'='PILL'
'makes' is the link.
(Other definitions for pill that I've seen before include "Medicinal pellet" , "Lozenge" , "Medical tablet" , "Form of medicine" , "nuisance to put up with" .)