Right to stop an ancient poet (6)
I believe the answer is:
arnold
'poet' is the definition.
(19th-century poet Matthew Arnold)
'right to stop an ancient' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to stop' means one lot of letters goes inside another.
'ancient' becomes 'old' (similar in meaning).
'r' going inside 'an' is 'arn'.
'arn'+'old'='ARNOLD'
(Other definitions for arnold that I've seen before include "Overcome (obstacle)" , "Sir Malcolm -, composer (Bridge over the River Kwai)" , "-- Wesker, playwright" , "Matthew --; -- Bennett" , "Victorian poet" .)