After a month, seek disguise (4)
I believe the answer is:
mask
'disguise' is the definition.
(masking is a kind of disguising)
'after a month seek' is the wordplay.
'after' is a charade indicator (letters next to each other).
'a month' becomes 'm' (standard abbreviation).
'seek' becomes 'ask' (I've seen this before).
'm'+'ask' is 'MASK'.
(Other definitions for mask that I've seen before include "Face covering; disguise" , "Visor; disguise" , "cloak" , "Facial disguise" , "Cover for the face" .)