Wax's opposite (4)
I believe the answer is:
wane
'wax's opposite' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wane that I've seen before include "Lose force, intensity" , "Dwindle" , "Grow smaller like the moon" , "Decline or go down" , "Gradual decline, of the moon say" .)