Barrier is, say, useful cover (8)
I believe the answer is:
handicap
'barrier' is the definition.
(I've seen this before)
'say useful cover' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'useful' becomes 'handy' (I've seen this before).
'cover' becomes 'cap' (cap is a kind of cover**).
'handy' sounds like 'handi'.
'handi'+'cap'='HANDICAP'
'is' acts as a link.
(Other definitions for handicap that I've seen before include "Put at a disadvantage" , "advantage - or the opposite" , "Impairment" , "Disadvantage to offset an advantage to equalise chances of winning" , "It makes progress difficult" .)