Lowest catch sealing insulated container (10)
I believe the answer is:
nethermost
'lowest' is the definition.
(I've seen this before)
'catch sealing insulated container' is the wordplay.
'catch' becomes 'net' (net can mean to catch).
'sealing' means one lot of letters goes inside another (inserted letters are sealed inside).
'insulated container' becomes 'thermos' (I've seen this in another clue).
'net' enclosing 'thermos' is 'NETHERMOST'.
(Other definitions for nethermost that I've seen before include "located in lowest position" , "at the bottom of the pile" .)