Wax's counterpart (4)
I believe the answer is:
wane
'wax's counterpart' 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 "Decline or go down" , "Become weaker" , "Atrophy" , "Diminish" , "Decrease, decline" .)