Fidelity of commission primarily requiring change of sides (7)
I believe the answer is:
'fidelity' is the definition.
(I know that fidelity can be written as loyalty)
I don't understand how the remainder of the clue works.
(Other definitions for loyalty that I've seen before include "Fidelity, allegiance" , "Faithfulness" , "Strong allegiance" , "Firmness of allegiance" , "Allegiance, commitment" .)