A hairstyle for each month (4)
I believe the answer is:
perm
'a hairstyle' is the definition.
(perm is a hairstyle)
'each month' is the wordplay.
'each' becomes 'per' ('per' can mean for each).
'month' becomes 'M' (standard abbreviation).
'per'+'m'='PERM'
'for' is the link.
(Other definitions for perm that I've seen before include "Old-fashioned haircut" , "Football pools forecast - wavy hairdo" , "Hair setting treatment; pools selection system" , "combo in pools" , "Russian port; hairstyle" .)