Bone discovered up in San Luis (4)
I believe the answer is:
ulna
'bone' is the definition.
(ulna is a kind of bone)
'up in san luis' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'in' indicates a hidden word.
'anlu' is hidden in the letters of 'san luis'.
'anlu' backwards is 'ULNA'.
'discovered' acts as a link.
(Other definitions for ulna that I've seen before include "Part of bone-structure" , "part of anatomy" , "Thinner of two long bones in forearm" , "Car document; ticket book" , "The longer and inner bone in forearm" .)