Mad about obstruction (3)
I believe the answer is:
dam
'obstruction' is the definition.
(I know that dam is a type of boom barrier)
'mad about' is the wordplay.
'about' says the letters should be written backwards.
'mad' back-to-front is 'DAM'.
(Other definitions for dam that I've seen before include "A mad kind of mum" , "Water retainer" , "Barrier to hold back water" , "Mother of quadruped" , "Embankment" .)