High room that ticks boxes (5)
I believe the answer is:
attic
'high room' is the definition.
(I've seen this in another clue)
'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 "Athenian Greek" , "Floor" , "Tacit (anag.)" , "from Greece" , "Lofty room" .)