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' says to take the initial letters (in a down clue, the first letter is top).
'unwell' becomes 'i'll' (similar in meaning).
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" .)