Royal house near castle mostly dilapidated (9)
I believe the answer is:
lancaster
'royal house' is the definition.
(Lancaster is an example)
'near castle mostly dilapidated' is the wordplay.
'mostly' means to remove the last letter.
'dilapidated' indicates an anagram.
'near'+'castle'='nearcastle'
'nearcastle' with its final letter removed is 'nearcastl'.
'nearcastl' with letters rearranged gives 'LANCASTER'.
(Other definitions for lancaster that I've seen before include "UK city" , "15th Century house" , "He appeared in many films" , "town > house" , "WW2 bomber; city" .)