Sticky material casing marrow in bones (5)
I believe the answer is:
tarsi
'bones' is the definition.
(bones of the foot)
'sticky material casing marrow' is the wordplay.
'sticky material' becomes 'tar' (tar is a sticky material).
'casing marrow' becomes 'si' (I can't justify this - if you can you should believe this answer much more).
'tar'+'si'='TARSI'
'in' is the link.
Can you help me to learn more?
(Other definitions for tarsi that I've seen before include "Parts of skeleton" , "parts of leg" , "collections of bones?" , "Foot bones" , "Bones of the ankles and heels" .)