Belabour a claim to be an accomplished party (6)
I believe the answer is:
overdo
'belabour a claim to' is the definition.
I can't tell whether this definition defines the answer.
'an accomplished party' is the wordplay.
'an accomplished' becomes 'over' (I've seen this before).
'party' becomes 'do' (do is a kind of party).
'over'+'do'='OVERDO'
'be' is the link.
Can you help me to learn more?
(Other definitions for overdo that I've seen before include "Cook wrongly" , "Sum of a persons actions (Hinduism)" , "Be excessive, perhaps cook too much" , "Exaggerate, go too far" , "Work too much" .)