What’s collapsed by spring in a heap — rot, sadly (5,3)
I believe the answer is:
'what's collapsed by spring' is the definition.
I can't judge whether this definition defines the answer.
'a heap rot sadly' is the wordplay.
'sadly' indicates anagramming the letters.
'aheaprot' with letters rearranged gives 'OPERA-HAT'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for opera hat that I've seen before include "Headgear to hear 14dn" , "garment in theatre?" , "Formal theatre wear" , "Lid that collapses" , "collapsible accessory - atop him surely?" .)