Small dog eating top of royal uniform’s wig (6)
I believe the answer is:
peruke
'wig' is the definition.
(I know that peruke is a type of wig)
'small dog eating top of royal uniform's' is the wordplay.
'small dog' becomes 'peke' (short for Pekingese; 'small' asks for a short form).
'eating' is an insertion indicator.
'top of' suggests taking the first letters.
The first letters of 'royal uniforms' is 'ru'.
'peke' placed around 'ru' is 'PERUKE'.
(Other definitions for peruke that I've seen before include "Periwig" , "Old fashionable wig" , "Ancient wig" , "18th-century wig" .)