A passing craze due to finish will weaken (4)
I believe the answer is:
fade
'weaken' is the definition.
(fading is a kind of weakening)
'a passing craze due to finish' is the wordplay.
'a passing craze' becomes 'fad' (I've seen this before).
'to finish' suggests the final letters.
The last letter of 'due' is 'e'.
'fad'+'e'='FADE'
'will' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for fade that I've seen before include "Lose intensity, colour" , "Wither away" , "Lose colour or vitality" , "Bleach or decline" , "Diminish" .)