Protection ahead of assault means an additional expense (5,6)
I believe the answer is:
cover charge
'an additional expense' is the definition.
(charge added to a bill)
'protection ahead of assault' is the wordplay.
'protection' becomes 'cover' (something that protects or shields).
'ahead of' means one lot of letters go next to another (some letters go in front of others).
'assault' becomes 'charge' (charging is a kind of assaulting).
'cover'+'charge'='COVER CHARGE'
'means' is the link.
(Other definitions for cover charge that I've seen before include "It's made by a restaurant" , "entrance fee?" .)