Secret royal houses in a past style (5)

I believe the answer is:

retro

I'm a little stuck... Click here to teach me more about this clue!

'a past style' is the definition.
The definition suggests a singular noun which matches the answer.

'secret royal houses in' is the wordplay.
'in' indicates a hidden word.
'RETRO' can be found hidden inside 'secret royal houses'.

Can you help me to learn more?

(Other definitions for retro that I've seen before include "Imitative of an older style" , "Fashion recalling the past" , "Of a past time" , "imitating past fashion" , "Imitating a bygone style" .)

I've seen this clue in The Guardian.

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