Not even fourth on the bed (5)
I believe the answer is:
futon
'the bed' is the definition.
(type of bed)
'not even fourth on' is the wordplay.
'not even' means one should take alternating letters (remove even-numbered letters).
The alternating letters of 'fourth' are 'fut'.
'fut'+'on'='FUTON'
(Other definitions for futon that I've seen before include "Mattress used as a bed or sofa" , "Kind of Japanese bed convertible to couch" , "Kind of mattress" , "A Japanese folding bed-chair" , "Japanese mattress with wooden frame used as bed" .)