Go on do, mostly united in order (8)
I believe the answer is:
continue
'go on' is the definition.
('continue' can be a synonym of 'go on')
'do mostly united in order' is the wordplay.
'do' becomes 'con'.
'mostly' means to remove the last letter.
'in order' indicates anagramming the letters (the letters in a new order).
'united' with its last letter taken off is 'unite'.
'unite' anagrammed gives 'tinue'.
'con'+'tinue'='CONTINUE'
(Other definitions for continue that I've seen before include "Carry on" , "Last" , "Resume" , "Proceed" , "Don't stop, keep going" .)