Counter leader of opposition party (5)
I believe the answer is:
disco
'party' is the definition.
(dance party)
'counter leader of opposition' is the wordplay.
'counter' becomes 'disc' (I've seen this before).
'leader of' suggests taking the first letters.
The first letter of 'opposition' is 'o'.
'disc'+'o'='DISCO'
(Other definitions for disco that I've seen before include "Dance hall - music by Donna Summer and the Bee Gees, say" , "Musical party" , "Place to dance to recorded music, in short" , "Style of pop music" , "Nightclub with recorded music, in short" .)