Trick party executive's leader (4)
I believe the answer is:
dupe
'trick' is the definition.
(I've seen this before)
'party executive's leader' is the wordplay.
'party' becomes 'dup' (Democratic Unionist Party).
'leader' says to take the initial letters.
The first letter of 'executives' is 'e'.
'dup'+'e'='DUPE'
(Other definitions for dupe that I've seen before include "Deceive, trick" , "Hoodwink, deceive" , "Pawn, mug" , "Deceive, make fool of" , "Gull" .)