Cover arranged to hold inert gas (5)
I believe the answer is:
sheet
'cover' is the definition.
(sheeting is a kind of covering)
'arranged to hold inert gas' is the wordplay.
'arranged' becomes 'set' (similar in meaning).
'to hold' indicates putting letters inside.
'inert gas' becomes 'He' (chemical symbol for helium, an inert gas).
'set' placed around 'he' is 'SHEET'.
(Other definitions for sheet that I've seen before include "newspaper" , "Single piece of paper" , "Bed cover" , "Piece of linen; ship's rope" , "Large piece of fabric" .)