Start eating, it’s falling apart! (9)
I believe the answer is:
instigate
'start' is the definition.
(I've seen this before)
'eating it's falling apart' is the wordplay.
'falling apart' indicates an anagram.
'eating'+'its'='eatingits'
'eatingits' with letters rearranged gives 'INSTIGATE'.
(Other definitions for instigate that I've seen before include "Bring about, initiate" , "Set off or stir up" , "Bring about by incitement - its eating (anag)" , "Incite - set in motion" , "I get saint to urge or incite" .)