Passing fashion’s energy to decline (4)
I believe the answer is:
fade
'decline' is the definition.
(fade can mean to die away or decline)
'passing fashion's energy' is the wordplay.
'passing fashion' becomes 'fad' (I've seen this before).
'energy' becomes 'e' (physics symbol as in E=mca).
'fad'+'e'='FADE'
'to' is the link.
(Other definitions for fade that I've seen before include "Discolour" , "Deaf (anag.)" , "Grow dull" , "Droop" , "Grow paler" .)