Report of major shock? (4)
I believe the answer is:
mane
'shock?' is the definition.
(both can mean a mass of hair)
'report of major' is the wordplay.
'report of' shows a homophone (sound like).
'major' becomes 'main' (both can mean important).
'main' sounds like 'MANE'.
(Other definitions for mane that I've seen before include "Hair (of lion)" , "Lion or horse's 10 Across" , "Animal neck-hair" , "Animal's neck hair" , "Name for hair on horse's neck" .)