Sensible sounding French cross-country runner (5)
I believe the answer is:
seine
'french cross-country runner' is the definition.
'Seine' can be an answer for 'runner' (river in France). I am not certain of the 'french cross-country' bit.
'sensible sounding' is the wordplay.
'sensible' becomes 'sane' (sane can mean sensible or reasonable).
'sounding' indicates a 'sounds like' (homophone) clue.
'sane' is a homophone of 'SEINE'.
Can you help me to learn more?
(Other definitions for seine that I've seen before include "Paris river" , "Parisian river" , "Paris's river" , "Second longest river in France" , "French river -- type of fishing net" .)