Flash and shock enveloping extremely large megaphone (10)
I believe the answer is:
loudhailer
'megaphone' is the definition.
(I've seen this before)
'flash and shock enveloping extremely large' is the wordplay.
'flash' becomes 'loud' ('loud' can be a synonym of 'flash'**).
'and' is a charade indicator (letters next to each other).
'shock' becomes 'hair' (I've seen this before).
'enveloping' is an insertion indicator.
'extremely' means to remove the middle letters.
'large' with its centre taken out is 'le'.
'hair' going around 'le' is 'hailer'.
'loud'+'hailer'='LOUDHAILER'
(Other definitions for loudhailer that I've seen before include "one gives sound more volume" , "Megaphone" .)