Is going into the mountain due to bad visibility (4)
I believe the answer is:
mist
'bad visibility' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'is going into the mountain' is the wordplay.
'is going' becomes 'is' (I've seen this in other clues).
'into' is an insertion indicator.
'the mountain' becomes 'mt'.
'is' put into 'mt' is 'MIST'.
'due to' is the link.
Can you help me to learn more?
(Other definitions for mist that I've seen before include "Watery vapour" , "low-level cloud" , "Haze" , "Thin layer of fog" , "Think fog or low cloud" .)