Rule reversed by European PM of old (7)
I believe the answer is:
walpole
'pm of old' is the definition.
The definition suggests a singular noun which matches the answer.
'rule reversed by european' is the wordplay.
'rule' becomes 'law' (law is a kind of rule).
'reversed' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'european' becomes 'pole' (someone from Poland).
'law' written backwards gives 'wal'.
'wal'+'pole'='WALPOLE'
Can you help me to learn more?
(Other definitions for walpole that I've seen before include "Horace -, Castle of Otranto author" , "English PM" , "First British PM" , "Robert ---, PM; Horace ---, novelist" , "Whig statesman" .)