Protective garment being worn after thirty days (5)
I believe the answer is:
apron
'protective garment' is the definition.
(apron is a kind of protective garment)
'worn after thirty days' is the wordplay.
'worn' becomes 'on'.
'after' means one lot of letters go next to another.
'thirty days' becomes 'Apr' (short for April).
'on' after 'apr' is 'APRON'.
'being' acts as a link.
(Other definitions for apron that I've seen before include "item of clothing in the kitchen" , "Garment for protection" , "Paved surface where aircraft stand when not in use" , "that's used by Oliver!?" , "Protective garment - part of stage" .)