### Clue: A topmost chamber starts to describe it, looking up? (5)
### Answer: ATTIC 
 ### Explanation: A topmost chamber starts to describe it, looking up? (5)
An envelope ('to describe') of TI, a reversal ('looking up' in a down light) of 'it' in ATC ('**A T**opmost **C**hamber starts'), with an extended definition.