CO manifested in muscle oxidation (4)
I believe the answer is:
cleo
'co manifested' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'in muscle oxidation' is the wordplay.
'in' indicates a hidden word.
'CLEO' is hidden in the letters of 'muscle oxidation'.
(Other definitions for cleo that I've seen before include "Carry on -- (1964 film)" , "She ruled" , "Woman" , "Ms Laine - one of the 14 films" , "Jazz singer, ___ Laine" .)