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