Two rivers enthralling the French nursery-rhyme twin (10)
I believe the answer is:
tweedledee
'nursery-rhyme twin' is the definition.
(twin of Tweedledum)
'two rivers enthralling the french' is the wordplay.
'two rivers' can mean two substitutions for 'river'.
'river' becomes 'tweed' (I've seen this before).
'river' becomes 'dee' (River Dee).
'enthralling' means one lot of letters goes inside another.
'the french' becomes 'le' ('the' in French).
'tweed'+'dee'='tweeddee'
'tweeddee' placed around 'le' is 'TWEEDLEDEE'.
(Other definitions for tweedledee that I've seen before include "Twin of 6 dn" , "aggressive twin" , "Twin met by Alice" , "Through the Looking Glass twin" .)