Turn, almost unable to see mischievous imp (6)
I believe the answer is:
goblin
'mischievous imp' is the definition.
'turn almost unable to see' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'almost' means to remove the last letter (most of the word but not all of it).
'unable to see' becomes 'blind' (I've seen this before).
'blind' with its last letter taken away is 'blin'.
'go'+'blin'='GOBLIN'
(Other definitions for goblin that I've seen before include "Mischievous ugly creature of folklore" , "Mischievous sprite" , "Small grotesque supernatural creature" , "Evil imp" , "mischievous spirit" .)