Party can become united - that's definitely on (6)
I believe the answer is:
doable
'united that's definitely on' is the definition.
I can't tell whether this defines the answer.
'party can' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'can' becomes 'able' (I've seen this before).
'do'+'able'='DOABLE'
'become' acts as a link.
Can you help me to learn more?
(Other definitions for doable that I've seen before include "easy to achieve" , "possible" , "Able to be carried out" , "Achievable, performable" .)