Excellence of French waters, exactly what’s needed (5)
I believe the answer is:
merit
'excellence' is the definition.
(I've seen this before)
'french waters exactly what's needed' is the wordplay.
'french waters' becomes 'mer' ('sea' in French).
'exactly what's needed' becomes 'it'.
'mer'+'it'='MERIT'
'of' acts as a link.
(Other definitions for merit that I've seen before include "Deserve; excellence" , "Excellence, desert" , "of a high order?" , "Excellence, worth" , "The M in OM" .)