Snake upset rook (7)
I believe the answer is:
rattler
'snake' is the definition.
(rattler is a kind of snake)
'upset rook' is the wordplay.
'upset' becomes 'rattle' (I've seen this before).
'rook' becomes 'r' (abbreviation in chess).
'rattle'+'r'='RATTLER'
(Other definitions for rattler that I've seen before include "Poisonous American" , "Nickname for US snake with horny segments in its tail" , "Rickety old vehicle" , "US snake (colloq.)" , "that makes one frightened" .)