Simple fellow is overturning my French (5)
I believe the answer is:
simon
'simple fellow' is the definition.
(as in the nursery rhyme character Simple Simon)
'is overturning my french' is the wordplay.
'overturning' is a reversal indicator.
'my french' becomes 'mon' ('my' in French).
'is' written backwards gives 'si'.
'si'+'mon'='SIMON'
(Other definitions for simon that I've seen before include "He's simple in rhyme" , "Fellow" , "A simple character in nursery rhyme" , "Apostle Peter's other name" , "Simple pieman" .)