Escape illness with drug (4)
I believe the answer is:
flue
'escape' is the definition.
(I've seen this before)
'illness with drug' is the wordplay.
'illness' becomes 'flu' (flu is a kind of health problem).
'with' means one lot of letters go next to another.
'drug' becomes 'e' (E is another name for ecstasy).
'flu'+'e'='FLUE'
(Other definitions for flue that I've seen before include "Fuel (anag.)" , "Tube" , "Passage for air" , "Escape" , "Vent pipe for smoke, etc" .)