Decreases in strength (5)
I believe the answer is:
wanes
'decreases in strength' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wanes that I've seen before include "Gradually declines" , "Decreases in vigour or strength like the moon" , "Of the moon, gradually decreases" , "Grows smaller like the moon" .)