Volume by Trump, in its local form, primarily authored by Moscow (8)
I believe the answer is:
capacity
'volume' is the definition.
(capacity is a kind of volume)
'trump in its local form primarily authored by moscow' is the wordplay.
I cannot really see how this works, but
'trump' could be 'cap' (both can mean to beat or override) and 'cap' is found within the answer.
'its' could be 'it' (I've seen this in other clues) and 'it' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'by' is the link.
Can you help me to learn more?
(Other definitions for capacity that I've seen before include "Ability to perform or produce" , "Amount that can be held - skill" , "Specified role or position" , "maximum size" , "The amount that can be contained, space" .)