Moves slowly in never-ending game (6)
I believe the answer is:
inches
'moves slowly' is the definition.
(I've seen this before)
'in never-ending game' is the wordplay.
'never ending' means to remove the last letter.
'game' becomes 'chess' (chess game is a kind of game).
'chess' with its last letter removed is 'ches'.
'in'+'ches'='INCHES'
(Other definitions for inches that I've seen before include "makes slow progress" , "Old units of length" , "Slowly makes one's way" , "Imperial units of length" , "Twelve of them in a foot" .)