Pretend to be grownups in building (9)
I believe the answer is:
playhouse
'grownups in building' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'pretend to' is the wordplay.
'pretend' becomes 'play' (playing is a kind of pretending).
'to' becomes 'house' (I can't explain this - if you can you should believe this answer much more).
'play'+'house'='PLAYHOUSE'
'be' is the link.
Can you help me to learn more?
(Other definitions for playhouse that I've seen before include "army theatre" , "Stage" .)