Upstairs room that ticks boxes (5)
I believe the answer is:
attic
'upstairs room' is the definition.
(I've seen this before)
'that ticks boxes' is the wordplay.
'boxes' indicates the answer is hidden within the clue (to box can mean to contain).
'ATTIC' can be found hidden inside 'that ticks'.
(Other definitions for attic that I've seen before include "Lofty room" , "Top apartment" , "Athenian Greek" , "Tacit (anag.)" , "The highest room" .)