Thirty days on aircraft parking area (5)
I believe the answer is:
apron
'aircraft parking area' is the definition.
(I've seen this before)
'thirty days on' is the wordplay.
'thirty days' becomes 'Apr' (short for April).
'apr'+'on'='APRON'
(Other definitions for apron that I've seen before include "Protective garment in kitchen" , "Aircraft parking area" , "standing" , "Protective garment - type of stage" , "Kitchen garment or airfield surface." .)