In spite of adequacy, weaken (4)
I believe the answer is:
fade
'weaken' is the definition.
(I know that fade is a more specific form of the action the weaken)
'in spite of adequacy' is the wordplay.
'in' says the answer is hidden in the clue.
'FADE' is hidden in the letters of 'spite of adequacy'.
(Other definitions for fade that I've seen before include "Lose brightness" , "Gradually grow faint" , "Lose colour gradually" , "Wither away" , "Lose strength" .)