Joke to admit Nationals leader to the work party (4)
I believe the answer is:
gang
'the work party' is the definition.
(synonyms)
'joke to admit nationals leader' is the wordplay.
'joke' becomes 'gag' (a gag is a joke or witticism).
'to admit' means one lot of letters goes inside another.
'leader' says to take the initial letters.
The initial letter of 'nationals' is 'n'.
'gag' going around 'n' is 'GANG'.
'to' is the link.
(Other definitions for gang that I've seen before include "Criminal mob" , "Informal body of friends" , "Association of criminals" , "Group of friends, criminals" , "Criminals collectively" .)