Muscle oil giant's keeping in reserve (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'oil giant's keeping in reserve' is the wordplay.
I cannot really see how this works, but
'reserve' could be 'ice' (both can mean aloofness) and 'ice' is present in the answer.
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 "I bend the elbow" , "Type of muscle" , "Forearm-flexing muscle" , "Elbow-bending muscle" , "One operating arm" .)