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' says the answer is hidden in 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 "Floor" , "Top storey" , "Small room at the top" , "Athenian Greek" , "room with lots of junk?" .)