Very scary, shock increasing (4-7)
I believe the answer is:
hair-raising
'very scary' is the definition.
(I've seen this in another clue)
'shock increasing' is the wordplay.
'shock' becomes 'hair' (shock can mean a mass of hair).
'increasing' becomes 'raising' (raising is a kind of increasing).
'hair'+'raising'='HAIR-RAISING'
(Other definitions for hair-raising that I've seen before include "Extremely alarming" , "Very frightening" , "Very scary" .)