Claw used in some brutal onslaught (5)
I believe the answer is:
talon
'claw' is the definition.
(talon is a kind of claw)
'in some brutal onslaught' is the wordplay.
'in' indicates a hidden word.
'TALON' is hidden within 'some brutal onslaught'.
'used' is the link.
(Other definitions for talon that I've seen before include "Raptor" , "The stock" , "a gripping one" , "One may be grabbing" , "Prey-bird's claw" .)