Hard appearance that could be a shock (4)
I believe the answer is:
hair
'a shock' is the definition.
(I've seen this before)
'hard appearance' is the wordplay.
'hard' becomes 'h'.
'appearance' becomes 'air' (I've seen this before).
'h'+'air'='HAIR'
'that could be' is the link.
(Other definitions for hair that I've seen before include "Showy garden flower" , "Show" , "Whisker" , "Thread-like strands on the body" , "Stage musical" .)