Ere leaving, feared it would be a passing fancy (3)
I believe the answer is:
fad
'a passing fancy' is the definition.
(a brief trend)
'ere leaving feared' is the wordplay.
'leaving' indicates named letters should be taken away.
'feared' with 'ere' taken away is 'FAD'.
'it would be' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for fad that I've seen before include "Craze; idiosyncrasy" , "A passing fashion or craze" , "Affectation" , "Personal whim" , "A craze or current vogue" .)