Role, key ultimately, for Labour? (5)
I believe the answer is:
party
'labour?' is the definition.
(the Labour Party is an example)
'role key ultimately' is the wordplay.
'role' becomes 'part' (synonyms).
'ultimately' suggests the final letters.
The last letter of 'key' is 'y'.
'part'+'y'='PARTY'
'for' acts as a link.
(Other definitions for party that I've seen before include "MP's faction" , "Crew" , "Festivity" , "Political group; function" , "Political group - celebration" .)