Ban hard water? It can be dangerous (5,3)
I believe the answer is:
black ice
'it can be dangerous' is the definition.
The definition suggests a singular noun which matches the answer.
'ban hard water?' is the wordplay.
'ban' becomes 'black' (I've seen this in other clues).
'hard water?' becomes 'ice' (I've seen this in another clue).
'black'+'ice'='BLACK ICE'
Can you help me to learn more?
(Other definitions for black ice that I've seen before include "on the surface, it's dodgy" , "Winter road hazard" , "Winter driving hazard" , "Dangerous road coating" .)