Exist with fraction of benefit (6)
I believe the answer is:
behalf
'benefit' is the definition.
(behalf is a kind of benefit)
'exist with fraction' is the wordplay.
'exist' becomes 'be' ('be' can be a synonym of 'exist').
'with' says to put letters next to each other.
'fraction' becomes 'half' (half is a kind of fraction).
'be'+'half'='BEHALF'
'of' is the link.
(Other definitions for behalf that I've seen before include "(On one's) account" , "Sake, interest, benefit" , "(A person's) interest" , "Part - account" , "(On someone's) account" .)