A hairstyle for every month (4)
I believe the answer is:
perm
'a hairstyle' is the definition.
(perm is a kind of hairstyle)
'every month' is the wordplay.
'every' becomes 'per' (I've seen this in other clues).
'month' becomes 'm' (standard abbreviation).
'per'+'m'='PERM'
'for' acts as a link.
(Other definitions for perm that I've seen before include "Football pools forecast - wavy hairdo" , "combo in pools" , "Hair setting method" , "locks far from straight?" , "Hairstyle treatment" .)