Republican rule upset American creature (6)
I believe the answer is:
walrus
'creature' is the definition.
'republican rule upset american' is the wordplay.
'republican' becomes 'R' (abbreviation in US politics).
'rule' becomes 'law' (law is a kind of rule).
'upset' is a reversal indicator.
'american' becomes 'US' (United States).
'r'+'law'='rlaw'
'rlaw' reversed gives 'walr'.
'walr'+'us'='WALRUS'
(Other definitions for walrus that I've seen before include "Large sea mammal" , "Seal's relative" , "Carpenter's companion?" , "'I am the ------', 1967 Lennon and McCartney song" , "Large tusked marine mammal" .)