Hard to leave Confucian classic, which is topping (5)
I believe the answer is:
icing
'is topping' is the definition.
(I've seen this before)
'hard to leave confucian classic' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to leave' suggests deleting specific letters.
'confucian classic' becomes 'I Ching' (ancient Chinese text).
'iching' with 'h' taken out is 'ICING'.
'which' is the link.
(Other definitions for icing that I've seen before include "Finishing cake" , "Sweet cake topping" , "You could see this on a cake" , "Sugar coating - a danger to aircraft, on roads etc" , "Food coating" .)