Bone marrow, usually in leg, is initially used up (5)
I believe the answer is:
ilium
'bone' is the definition.
(ilium is a kind of bone)
'marrow usually in leg is initially used up' is the wordplay.
'is initially' says to take the initial letters.
'used' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The first letters of 'marrow usually in leg' is 'muil'.
'muil'+'i'='muili'
'muili' backwards is 'ILIUM'.
Can you help me to learn more?
(Other definitions for ilium that I've seen before include "Large pelvic bone" , "Hipbone" , "Ancient city" , "Part of skeleton" , "classical city" .)