Hard look causing shock (4)
I believe the answer is:
hair
'shock' is the definition.
(shock can mean a mass of hair)
'hard look' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'look' becomes 'air' (something might have a certain air or look).
'h'+'air'='HAIR'
'causing' acts as a link.
(Other definitions for hair that I've seen before include "Famous hirsute musical with song ` `Aquarius' ' (4)" , "shock" , "Thread-like strands on the body" , "Human head covering" , "Famous hippy musical of hirsute character" .)