That tearful impediment of vision due to some mistake? (4)
I believe the answer is:
mist
'that tearful impediment' is the definition.
I can't tell whether this defines the answer.
'vision due to some mistake?' is the wordplay.
'vision due' becomes 'mis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to some' indicates the central letters.
The middle letter of 'mistake' is 't'.
'mis'+'t'='MIST'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for mist that I've seen before include "Haze" , "Cloud; thin fog" , "low-level cloud" , "Think fog or low cloud" , "Obscure" .)