Moves slowly during unfinished game (6)
I believe the answer is:
inches
'moves slowly' is the definition.
(I've seen this before)
'during unfinished game' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'unfinished' means to remove the last letter.
'game' becomes 'chess' (chess is a kind of game).
'chess' with its final letter taken away is 'ches'.
'in'+'ches'='INCHES'
(Other definitions for inches that I've seen before include "Twelve of them in a foot" , "Edges slowly" , "Twelves of these in a foot" , "Niches (anag.)" , "Scottish islands" .)