Voltaire, say - deep cynic lost at sea (13)
I believe the answer is:
encyclopedist
'voltaire' is the definition.
'deep cynic lost at sea' is the wordplay.
'at sea' is an anagram indicator.
'deep'+'cynic'+'lost'='deepcyniclost'
'deepcyniclost' with letters rearranged gives 'ENCYCLOPEDIST'.
'say' is the link.
(Other definitions for encyclopedist that I've seen before include "Large reference book contributor" , "Contributor to Britannica, Larousse etc" .)