Upstart beginning to clad royal house (6)
I believe the answer is:
stuart
'royal house' is the definition.
(royal house of Scotland and England)
'upstart beginning to clad' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'beginning' becomes 'start' (synonyms).
'to clad' means one lot of letters goes inside another (clad can mean covered by).
'u' going inside 'start' is 'STUART'.
(Other definitions for stuart that I've seen before include "C17 royal house" , "9 [HOUSE]" , "Royal family up to seventeen fourteen" , "House that used to lead" , "Scottish royal family" .)