Burrowing creature ruins the marrow (9)
I believe the answer is:
earthworm
'burrowing creature' is the definition.
(I've seen this before)
'ruins the marrow' is the wordplay.
'ruins' indicates anagramming the letters.
'the'+'marrow'='themarrow'
'themarrow' is an anagram of 'EARTHWORM'.
(Other definitions for earthworm that I've seen before include "Soil dweller" , "that's throwing up on the lawn" , "Tunneller" , "Garden burrower" , "One burrows" .)