Historian covering royal uniform (6)
I believe the answer is:
livery
'uniform' is the definition.
(livery is a type of formal uniform)
'historian covering royal' is the wordplay.
'historian' becomes 'Livy' (Roman historian).
'covering' is an insertion indicator.
'royal' becomes 'er' (abbreviation for Elizabeth Regina).
'livy' placed around 'er' is 'LIVERY'.
(Other definitions for livery that I've seen before include "Uniform worn by male servants" , "Uniform, eg of guild" , "The stabling and care of horses" , "Feeding and stabling of horses for money" , "Uniform - commercial care of horses" .)