Sign up again (8)
I believe the answer is:
reenlist
'sign up again' is the definition.
('enlist' is an answer for 'sign up' so perhaps 'reenlist' is OK for 'sign up again')
This is all the clue.
(Other definitions for reenlist that I've seen before include "join the army again?" , "Join up again in Leinster" , "sign up for duty again" .)