Restraint bound to secure extra cover (8)
I believe the answer is:
reinsure
'secure extra cover' is the definition.
(insure again)
'restraint bound' is the wordplay.
'restraint' becomes 'rein' (I've seen this before).
'bound' becomes 'sure' (synonyms).
'rein'+'sure'='REINSURE'
'to' acts as a link.
(Other definitions for reinsure that I've seen before include "Pass on a risk of having to pay compensation" , "Take out another policy" , "Cover again" , "recover?" .)