Building obstacle (5)
I believe the answer is:
block
This might be a double definition.
'building' is the first definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'obstacle' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for block that I've seen before include "Jam" , "City area; prevent flow" , "Mass of wood etc - set of buildings" , "Cube" , "cake?" .)