Vertical space in building Rita's altered thoroughly (9)
I believe the answer is:
stairwell
'vertical space in building' is the definition.
(space for a staircase)
'rita's altered thoroughly' is the wordplay.
'altered' indicates anagramming the letters.
'thoroughly' becomes 'well' (I've seen this in another clue).
'ritas' with letters rearranged gives 'stair'.
'stair'+'well'='STAIRWELL'
(Other definitions for stairwell that I've seen before include "where flight is located" , "flight area" , "Shaft for walking between floors" , "where flight goes" , "Space flight" .)