Monster in building or gone? (6)
I believe the answer is:
gorgon
'monster' is the definition.
(Gorgon is an example)
'in building or gone?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GORGON' is hidden in the letters of 'building or gone'.
(Other definitions for gorgon that I've seen before include "Unpleasant woman" , "She had the power to turn people to stone" , "In Greek myth, her glance turned people to stone" , "Female monster of Greek legend" , "In Greek myth, a snake-haired woman whose look turned watcher to stone" .)