Defence group, old, sat, hidden (9)

I believe the answer is:

unexposed

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'hidden' is the definition.
(I've seen this before)

'defence group old sat' is the wordplay.
'defence group' becomes 'un' (I am not sure about this - if you are sure you should believe this answer much more).
'old' becomes 'ex' (prefix meaning former or onetime).
'sat' becomes 'posed' ('pose' can be a synonym of 'sit').
'un'+'ex'+'posed'='UNEXPOSED'

Can you help me to learn more?

(Other definitions for unexposed that I've seen before include "fully clothed?" , "Roundish handle" , "Closet" , "film not used" , "Hidden" .)

I've seen this clue in The Daily Express.

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