Startled deer eats old crumble (5)
I believe the answer is:
erode
'crumble' is the definition.
(I know that erode is a more specific form of crumble)
'startled deer eats old' is the wordplay.
'startled' is an anagram indicator.
'eats' means one lot of letters goes inside another.
'old' becomes 'o' (abbreviation).
'deer' with letters rearranged gives 'erde'.
'erde' enclosing 'o' is 'ERODE'.
(Other definitions for erode that I've seen before include "Abrade" , "Corrode" , "Grind down" , "Wear away with weather" , "fragment" .)