In merriment see to mattress (5)
I believe the answer is:
futon
'mattress' is the definition.
(futon is a kind of mattress)
'in merriment see to' is the wordplay.
'in' indicates putting letters inside.
'merriment' becomes 'fun' ('fun' can be a synonym of 'merriment').
'fun' going around 'to' is 'FUTON'.
(Other definitions for futon that I've seen before include "Japanese floor-mattress" , "A Japanese folding bed-chair" , "Jap. quilt bed" , "Japanese-style mattress" , "Japanese mattress with frame, a bed-couch" .)