Everyone’s on one — packed in old fashioned mattress (9)
I believe the answer is:
palliasse
'mattress' is the definition.
(palliasse is a kind of mattress)
'everyone's on one packed in old' is the wordplay.
'everyone' becomes 'all' (all people).
'on' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'packed in' is an insertion indicator.
'old' becomes 'passe' (I've seen this in another clue).
'all'+'i'='alli'
'alli' inserted into 'passe' is 'PALLIASSE'.
'fashioned' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for palliasse that I've seen before include "mattress" , "where some might lie with comfort" , "Bedding" , "one would take lying down?" .)