Bones served up in mean lunch (5)
I believe the answer is:
ulnae
'bones' is the definition.
(ulna is a kind of bone)
'up in mean lunch' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' indicates a hidden word.
'eanlu' can be found hidden inside 'mean lunch'.
'eanlu' back-to-front is 'ULNAE'.
'served' acts as a link.
(Other definitions for ulnae that I've seen before include "parts of limb" , "Thin bones in upper arms" , "Army parts" .)