It holds the unloved Emperor motionless (5)
I believe the answer is:
inert
'motionless' is the definition.
(inertia is a lack of movement)
'it holds the unloved emperor' is the wordplay.
'holds' is an insertion indicator.
'the unloved emperor' becomes 'ner' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'it' enclosing 'ner' is 'INERT'.
Can you help me to learn more?
(Other definitions for inert that I've seen before include "Motionless or lifeless" , "Incapable of acting, like some gas" , "lazy" , "(Chem.) not reactive" , "Not active or reactive" .)