A trust broken in royal house (6)
I believe the answer is:
stuart
'royal house' is the definition.
(Stuart is an example)
'a trust broken in' is the wordplay.
'broken' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'trust' with letters rearranged gives 'sturt'.
'a' inserted inside 'sturt' is 'STUART'.
(Other definitions for stuart that I've seen before include "House that used to lead" , "Royal family up to seventeen fourteen" , "9 [HOUSE]" , "Scottish royal family" , "C17 royal house" .)