Room in roof that ticks boxes (5)
I believe the answer is:
attic
'room in roof' is the definition.
(I've seen this before)
'that ticks boxes' is the wordplay.
'boxes' indicates a hidden word (to box can mean to contain).
'ATTIC' is hidden in the letters of 'that ticks'.
(Other definitions for attic that I've seen before include "Floor" , "Top storey" , "Small room at the top" , "Athenian Greek" , "room with lots of junk?" .)