Wrong French river for this reptile (8)
I believe the answer is:
tortoise
'this reptile' is the definition.
(tortoise is a kind of reptile)
'wrong french river' is the wordplay.
'wrong' becomes 'tort' (legal term).
'french river' becomes 'oise'.
'tort'+'oise'='TORTOISE'
'for' acts as a link.
(Other definitions for tortoise that I've seen before include "Expected to be slow" , "Slow-moving four-footed reptile - outraced the hare" , "Might this animal turn to Tories?" , "Mobile home owner?" , "One that kept leading Achilles" .)