A mega wave (4)
I believe the answer is:
perm
'wave' is the definition.
(I know that perm is a type of wave)
'a mega' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'mega' becomes 'm' (this could be a standard abbreviation which I don't know about).
'per'+'m'='PERM'
Can you help me to learn more?
(Other definitions for perm that I've seen before include "Old-fashioned haircut" , "make waves" , "combo in pools" , "Russian city and hairstyle, in short" , "Hair setting treatment; pools selection system" .)