Hero declines portion of crumble (5)
I believe the answer is:
erode
'crumble' is the definition.
(I've seen this in another clue)
'hero declines portion of' is the wordplay.
'portion of' says the answer is hidden in the clue.
'ERODE' is hidden in the letters of 'hero declines'.
(Other definitions for erode that I've seen before include "Gradually wear away" , "'Deteriorate, wear away (5)'" , "go to pieces!" , "Become ground down or deteriorate" , "Grind down" .)