Become unable to see reduced mischievous sprite (6)
I believe the answer is:
goblin
'mischievous sprite' is the definition.
(I've seen this before)
'become unable to see reduced' is the wordplay.
'become' becomes 'go' ('go' can be a synonym of 'become').
'unable to see' becomes 'blind' (I've seen this before).
'reduced' means to remove the last letter.
'blind' with its last letter taken off is 'blin'.
'go'+'blin'='GOBLIN'
(Other definitions for goblin that I've seen before include "Small grotesque supernatural creature" , "Malevolent imp" , "Dwarf-like creature" , "I may appear in fairy tales" , "Small trouble-making creature of folklore" .)