Reference book (13)
'reference book' is the definition.
(encyclopaedia is a kind of reference book)
This is the entire clue.
(Other definitions for encyclopaedia that I've seen before include "source of information" , "Large reference book" , "Large book of general knowledge" , "Extensive reference work" , "Comprehensive work of reference arranged alphabetically" .)
(Other definitions for whos who that I've seen before include "Directory of notables" , "short lives" , "Directory of people" , "Top people's directory" , "Top-people book" .)