Small room near top of tower (6)
I believe the answer is:
closet
'small room' is the definition.
(I've seen this before)
'near top of tower' is the wordplay.
'near' becomes 'close' ('close' can be a synonym of 'near').
'top of' indicates taking the first letters.
The first letter of 'tower' is 't'.
'close'+'t'='CLOSET'
(Other definitions for closet that I've seen before include "US press for clothing" , "secret is revealed" , "Private room; cupboard" , "Covert" , "Compartment for clothing in the US" .)