One somehow sealing off very hot chamber (4)
I believe the answer is:
oven
'hot chamber' is the definition.
(I've seen this in another clue)
'one somehow sealing off very' is the wordplay.
'somehow' indicates anagramming the letters.
'sealing off' indicates putting letters inside (inserted letters are sealed inside).
'very' becomes 'v' (abbreviation).
'one' anagrammed gives 'oen'.
'oen' going around 'v' is 'OVEN'.
(Other definitions for oven that I've seen before include "Liquid oxygen; salmon" , "Small kiln" , "Cooking appliance" , "Compartment for heating" , "Metal box in which to roast" .)