A foreign affair involving drugs is not sensible! (9)
I believe the answer is:
unfeeling
'not sensible' is the definition.
(I know that sensible is the opposite of unfeeling)
'a foreign affair involving drugs' is the wordplay.
'a foreign' becomes 'un' ('a' translated into French).
'affair' becomes 'fling' (a fling is a romantic affair).
'involving' indicates putting letters inside.
'drugs' becomes 'ee' (an abbreviation for ecstasy written twice).
'fling' placed around 'ee' is 'feeling'.
'un'+'feeling'='UNFEELING'
'is' acts as a link.
(Other definitions for unfeeling that I've seen before include "Hard-hearted" , "Heartless" , "Callous" , "Lacking in sympathy or emotion" .)