Bone set as rust forms (6)
I believe the answer is:
tarsus
'bone set' is the definition.
(group of bones)
'as rust forms' is the wordplay.
'forms' indicates an anagram.
'as'+'rust'='asrust'
'asrust' anagrammed gives 'TARSUS'.
(Other definitions for tarsus that I've seen before include "ancient Turkish city" , "City of which St Paul was a native" , "saint's birthplace" , "Saul of ......" , "The seven bones of the upper foot and ankle" .)