What an expert knows related in special paper (5,4)
I believe the answer is:
onion skin
'special paper' is the definition.
(very thin paper)
'what an expert knows related' is the wordplay.
'what an expert knows' becomes 'onions' (as in the phrase 'to know one's onions').
'related' becomes 'kin' (similar in meaning).
'onions'+'kin'='ONION-SKIN'
'in' is the link.
(Other definitions for onion skin that I've seen before include "a cut through which may make one cry" , "The sort of paper" , "whose content may make us weep!" , "Thin paper" .)