I concealed pay when working for a good reference (13)
I believe the answer is:
encyclopaedia
'a good reference' is the definition.
(reference work)
'i concealed pay when working' is the wordplay.
'when working' indicates anagramming the letters (the letters must work themselves into a new order).
'i'+'concealed'+'pay'='iconcealedpay'
'iconcealedpay' anagrammed gives 'ENCYCLOPAEDIA'.
'for' acts as a link.
(Other definitions for encyclopaedia that I've seen before include "Large reference work" , "Comprehensive work of reference arranged alphabetically" , "Extensive reference work" , "Large book of general knowledge" , "Large reference book" .)