Fabric in room is about to be put on ceiling (7)

'fabric' is the definition.
(challis is a kind of fabric)

'room is about to be put on ceiling' is the wordplay.
I cannot quite understand how this works, but
'room' could be 'hall' (hall is a kind of room) and 'hall' is found in the answer.
'is' is within the answer.
'about' could be 'c' (abbreviation for 'circa') and 'c' is found in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).

'in' acts as a link.

Can you help me to learn more?