Mad about obstacle (3)
I believe the answer is:
dam
'obstacle' is the definition.
(I know that dam is a type of obstacle)
'mad about' is the wordplay.
'about' is a reversal indicator.
'mad' in reverse letter order is 'DAM'.
(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" .)