Partly useful — narrow like some bones (5)
I believe the answer is:
ulnar
'like some bones' is the definition.
'partly useful narrow' is the wordplay.
'partly' indicates a hidden word.
'ULNAR' is hidden in the letters of 'useful narrow'.
(Other definitions for ulnar that I've seen before include "Relating to a particular arm bone" , "Relating to one of the bones of the forearm" , "Relating to a bone in the forearm" , "of certain bones" .)