Partly enable a guest to form alliance (6)
I believe the answer is:
league
'alliance' is the definition.
(I've seen this before)
'partly enable a guest' is the wordplay.
'partly' says the answer is hidden in the clue (part of the clue is used).
'LEAGUE' is hidden within 'enable a guest'.
'to form' is the link.
(Other definitions for league that I've seen before include "Group in football" , "Combination; three miles" , "Association of sports teams to organise matches" , "Union - measure" , "Association for mutual co-operation" .)