With a passing fancy, each approach is to disappear with time (4,4)
I believe the answer is:
fade away
'to disappear with time' is the definition.
(to slowly vanish over time)
'with a passing fancy each approach' is the wordplay.
'with' means one lot of letters go next to another.
'a passing fancy' becomes 'fad' (I've seen this before).
'each' becomes 'ea'.
'approach' becomes 'way'.
'fad'+'ea' is 'fadea'.
'fadea'+'way'='FADE-AWAY'
'is' is the link.
(Other definitions for fade away that I've seen before include "Flag" , "Disappear gradually" , "Die gradually" , "Become weaker" , "as did the Cheshire Cat?" .)