In Cairo Dutch family backing Old Royal House (5)
I believe the answer is:
tudor
'old royal house' is the definition.
(I've seen this in another clue)
'in cairo dutch family backing' is the wordplay.
'in' indicates a hidden word.
'backing' says the letters should be written backwards.
'rodut' can be found hidden inside 'cairo dutch family'.
'rodut' written backwards gives 'TUDOR'.
(Other definitions for tudor that I've seen before include "Elizabeth I of England was of this royal family" , "Name of English royal family during 16th century" , "Welsh royal family" , "Henry 7th's dynasty" , "Rose" .)