Almost become unable to see mischievous spirit (6)
I believe the answer is:
goblin
'mischievous spirit' is the definition.
(I've seen this before)
'almost become unable to see' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'become' becomes 'go' (synonyms).
'unable to see' becomes 'blind' (I've seen this before).
'go'+'blind'='goblind'
'goblind' with its final letter taken off is 'GOBLIN'.
(Other definitions for goblin that I've seen before include "Mischievous sprite" , "Malevolent gnome" , "Little monster" , "I may appear in fairy tales" , "Small trouble-making creature of folklore" .)