Begin royal house when banishing first of usurpers (5)
I believe the answer is:
start
'begin' is the definition.
(I know that begin can be written as start)
'royal house when banishing first of usurpers' is the wordplay.
'royal house' becomes 'Stuart' (royal house of Scotland and England).
'when banishing' indicates named letters should be taken away.
'first of usurpers' becomes 'u' (1st letter of 'usurpers').
'stuart' with 'u' taken away is 'START'.
(Other definitions for start that I've seen before include "Initiate" , "Onset" , "Beginning" , "Begin - jump" , "Move quickly < when surprised" .)