Mad about obstacle (3)
I believe the answer is:
dam
'obstacle' 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?
'mad about' is the wordplay.
'about' is a reversal indicator.
'mad' back-to-front is 'DAM'.
Can you help me to learn more?
(Other definitions for dam that I've seen before include "Barrier to contain the flow of water" , "Mother of quadruped" , "Mother - barrier" , "Water barrier, quite mad" , "River blockage" .)