English politician to go wrong entertaining old ruler (7)
I believe the answer is:
emperor
'ruler' is the definition.
(I've seen this before)
'english politician to go wrong entertaining old' is the wordplay.
'english' becomes 'e' (abbreviation).
'politician' becomes 'mp'.
'to go wrong' becomes 'err' (I've seen this before).
'entertaining' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'err' enclosing 'o' is 'eror'.
'e'+'mp'+'eror'='EMPEROR'
(Other definitions for emperor that I've seen before include "noted by Beethoven" , "Kaiser" , "Beethoven piano concerto, Op. 73" , "Monarch" , "Ruler of large territory" .)