Beavers' construction (3)
I believe the answer is:
dam
'beavers' construction' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dam that I've seen before include "Water barrier, quite mad" , "Horse's mother" , "Embankment" , "A mad sort of mum" , "Mad as an animal mum" .)