French painter's uncanny sea mist (7)
I believe the answer is:
matisse
'french painter's' is the definition.
(I've seen this before)
'uncanny sea mist' is the wordplay.
'uncanny' indicates an anagram.
'sea'+'mist'='seamist'
'seamist' with letters rearranged gives 'MATISSE'.
(Other definitions for matisse that I've seen before include "20th century French painter" , "Leading fauvist painter, d. 1954" , "Is mates turned on the French painter" , "Henri -, nineteenth-twentieth century French painter/sculptor" , "French impressionist" .)