Useful exterior of sodium and bone (4)
I believe the answer is:
ulna
'bone' is the definition.
(ulna is a kind of bone)
'useful exterior of sodium' is the wordplay.
'exterior of' says to hollow out the word (remove centre letters).
'sodium' becomes 'NA' (Na is the chemical symbol for sodium).
'useful' with its middle removed is 'ul'.
'ul'+'na'='ULNA'
'and' acts as a link.
(Other definitions for ulna that I've seen before include "Long bone in the arm" , "Uneven" , "One of two bones of the forearm" , "Bone from the human elbow" , "Thinner and longer of two bones in forearm" .)