When heavyweight is hard to shock (8)
I believe the answer is:
astonish
'shock' is the definition.
(I've seen this before)
'when heavyweight is hard' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'heavyweight' becomes 'ton' (a ton weighs a lot).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'as'+'ton'+'is'+'h'='ASTONISH'
'to' is the link.
(Other definitions for astonish that I've seen before include "Dumbfound" , "Shock with surprise" , "Flabbergast" , "Amaze, astound" , "Floor" .)