Shin up, Everyman? In a bit (5)
I believe the answer is:
tibia
'shin' is the definition.
(synonyms)
'up everyman? in a bit' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'everyman?' becomes 'i' (where 'Everyman' is the setter of the crossword).
'in' means one lot of letters goes inside another.
'a'+'bit'='abit'
'i' placed within 'abit' is 'aibit'.
'aibit' back-to-front is 'TIBIA'.
(Other definitions for tibia that I've seen before include "Bone of the lower leg" , "The formal shinbone" , "Shin bone" , "part of skeleton" , "Part of leg of" .)