A French fee that's excessive (5)
I believe the answer is:
undue
'excessive' is the definition.
(I know that excessive can be written as undue)
'a french fee' is the wordplay.
'a french' becomes 'un' ('a' in French).
'fee' becomes 'due' (I am not sure about this - if you are sure you should believe this answer much more).
'un'+'due'='UNDUE'
'that's' is the link.
Can you help me to learn more?
(Other definitions for undue that I've seen before include "too much" , "Needless; excessive" , "Excessively, unreasonably" , "Improper" , "Excessive, unreasonable" .)