Beast has right law backing us (6)
I believe the answer is:
walrus
'beast' is the definition.
(I've seen this before)
'right law backing us' is the wordplay.
'right' becomes 'r' (common abbreviation).
'backing' says the letters should be written backwards.
'r'+'law'='rlaw'
'rlaw' reversed gives 'walr'.
'walr'+'us'='WALRUS'
'has' is the link.
(Other definitions for walrus that I've seen before include "'The time has come', the . . . said" , "'I am the ------', 1967 Lennon and McCartney song" , "Arctic dwellers" , "Tusked marine animal" , "Marine mammal with ivory tusks and a moustache" .)