Critic beginning to rattle American relations (6)
I believe the answer is:
ruskin
'critic' is the definition.
(I've seen this before)
'beginning to rattle american relations' is the wordplay.
'beginning to' says to take the initial letters.
'american' becomes 'us'.
'relations' becomes 'kin' (I've seen this before**).
The initial letter of 'rattle' is 'r'.
'r'+'us'+'kin'='RUSKIN'
(Other definitions for ruskin that I've seen before include "English author" , "John --. C19 art and social critic" , "Victorian writer" , "John --, English art critic and reformer" , "John -, English art and social critic" .)