A passing craze due to finish will weaken (4)
I believe the answer is:
Here is my best explanation:
'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'.
'will' is the link.
I am not very happy about this link. It may belong to another bit of the clue.