Discerning writer and French sailor (11)
I believe the answer is:
penetrating
'discerning' is the definition.
(thesaurus)
'writer and french sailor' is the wordplay.
'writer' becomes 'pen' (something which writes).
'and french' becomes 'et' ('and' in French).
'sailor' becomes 'rating' (British term for a naval enlisted man).
'pen'+'et'+'rating'='PENETRATING'
(Other definitions for penetrating that I've seen before include "Acute" , "Boring - keen" , "Incisive" , "Going through" .)