It preserves well-concealed architectural feature, they say (4,6)

I believe the answer is:

deep freeze

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'it preserves' is the definition.
The definition suggests a singular noun but the answer is not.

'well-concealed architectural feature they say' is the wordplay.
'well concealed' becomes 'deep' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'architectural feature' becomes 'frieze' (I've seen this before).
'they say' shows a homophone (sound like).
'frieze' sounds like 'freeze'.
'deep'+'freeze'='DEEP-FREEZE'

Can you help me to learn more?

(Another definition for deep freeze that I've seen is " Food store".)

I've seen this clue in The Guardian.

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