River crustacean the French must find dreadful (9)
I believe the answer is:
execrable
'dreadful' is the definition.
(I've seen this before)
'river crustacean the french' is the wordplay.
'river' becomes 'exe' (River Exe).
'crustacean' becomes 'crab' (crab is a kind of crustacean).
'the french' becomes 'le' ('the' in French).
'exe'+'crab'+'le'='EXECRABLE'
'must find' acts as a link.
(Other definitions for execrable that I've seen before include "Extremely bad or unpleasant" , "Accursed" , "Abominable, detestable" , "Loathsome" , "A celeb rex may be abominable" .)