Organ's terrible scare after backing racing certainty (8)
I believe the answer is:
pancreas
'organ's' is the definition.
(pancreas is a kind of organ)
'terrible scare after backing racing certainty' is the wordplay.
'terrible' indicates an anagram.
'after' means one lot of letters go next to another.
'backing' says the letters should be written backwards.
'racing certainty' becomes 'nap' (I've seen this before).
'nap' reversed gives 'pan'.
'scare' with letters rearranged gives 'creas'.
'creas' after 'pan' is 'PANCREAS'.
(Other definitions for pancreas that I've seen before include "Gland aiding digestion" , "Part of body" , "secreting structure" , "Gland secreting digestive enzymes" , "insulin producer" .)