It rearranges soil when the marrow is loosened (9)
I believe the answer is:
earthworm
'it rearranges soil' is the definition.
(earthworms burrow through soil)
'the marrow is loosened' is the wordplay.
'is loosened' indicates anagramming the letters.
'the'+'marrow'='themarrow'
'themarrow' with letters rearranged gives 'EARTHWORM'.
'when' is the link.
(Other definitions for earthworm that I've seen before include "Soil-dwelling creature" , "Soil-dwelling invertebrate" , "Garden burrower" , "Tunneller" , "Creature that burrows in soil" .)