Restriction certainly needed for cover again (8)
I believe the answer is:
reinsure
'cover again' is the definition.
('insure' can be a synonym for 'cover' so perhaps 'reinsure' is OK for 'cover again')
'restriction certainly needed' is the wordplay.
'restriction' becomes 'rein' (I've seen this before).
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly').
'needed' means one lot of letters go next to another.
'rein'+'sure'='REINSURE'
'for' is the link.
(Other definitions for reinsure that I've seen before include "Cover again" , "Pass on a risk of having to pay compensation" , "Take out another policy" , "recover?" .)