Royal family's impromptu do retainer conceals (5)
I believe the answer is:
tudor
'royal family's' is the definition.
(Tudor is an example)
'impromptu do retainer conceals' is the wordplay.
'conceals' says the answer is hidden in the clue.
'TUDOR' can be found hidden inside 'impromptu do retainer'.
(Other definitions for tudor that I've seen before include "Welsh royal house" , "Elizabeth I of England was of this royal family" , "Half-timbered-house style" , "Name of a royal house" , "Elizabeth I, perhaps" .)