Fool in Belfast party taking recreational drug (4)
I believe the answer is:
dupe
'fool' is the definition.
(I know that fool can be written as dupe)
'belfast party taking recreational drug' is the wordplay.
'belfast party' becomes 'DUP' (Democratic Unionist Party, Northern Irish party).
'taking' means one lot of letters go next to another.
'recreational drug' becomes 'e' (E can refer to ecstasy).
'dup'+'e'='DUPE'
'in' acts as a link.
(Other definitions for dupe that I've seen before include "Cheat" , "Gull" , "Fool or hoax" , "Victim of deceit or fraud" , "Cat's paw" .)