Plums sadly in collapse (5)
I believe the answer is:
slump
'collapse' is the definition.
(slumping is a kind of collapsing)
'plums sadly' is the wordplay.
'sadly' indicates an anagram.
'plums' anagrammed gives 'SLUMP'.
'in' acts as a link.
(Other definitions for slump that I've seen before include "Heavy fall" , "Sudden drop in business" , "Bad economic recession" , "Commercial decline" , "Sudden fall as in share values" .)