Become weaker (4)
(Other definitions for fade that I've seen before include "Die away" , "Lose colour gradually" , "Droop" , "Gradually disappear from view" , "Discolour" .)
'become weaker' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for wane that I've seen before include "Grow smaller like the moon" , "Diminish" , "Get smaller" , "Become weaker" , "Decline (in influence)" .)