Oil spillage concerning the French river (5)
I believe the answer is:
loire
'french river' is the definition.
(River Loire)
'oil spillage concerning' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'oil' is 'loi' which is present in the answer.
'concerning' could be 're' (both can mean on a particular topic) and 're' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'the' acts as a link.
Can you help me to learn more?
(Other definitions for loire that I've seen before include "River of central France" , "French department" , "France's longest river" , "Longest river in France" , "Wine region" .)