Black spice in mixture builds muscle (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'black spice in mixture builds' is the wordplay.
I cannot really understand how this works, but
'black' could be 'b' (abbreviation as in pencil hardness) and 'b' is present in the answer.
an anagram of 'spice' is 'iceps' which is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for biceps that I've seen before include "Upper-arm muscle" , "Muscles on upper arms" , "Elbow-bending muscle" , "Type of muscle" , "Forearm-flexing muscle" .)