Mad about a barrier (3)
I believe the answer is:
dam
'a barrier' is the definition.
(dam is a kind of barrier)
'mad about' is the wordplay.
'about' shows that the letters should be reversed in order.
'mad' written backwards gives 'DAM'.
(Other definitions for dam that I've seen before include "Horse's mother" , "Block (river)" , "Female parent; obstruction" , "Mad as an animal mum" , "Reservoir barrier" .)