In our opinion the French mess up (6)
I believe the answer is:
tousle
'mess up' is the definition.
(I've seen this before)
'in our opinion the french' is the wordplay.
'in our opinion' becomes 'to us' (to us).
'the french' becomes 'le' ('the' in French).
'tous'+'le'='TOUSLE'
(Other definitions for tousle that I've seen before include "Treat roughly" , "Rumple, make untidy as with hair" , "Solute (anag)" , "Mess around" , "Ruffle (the hair)" .)