Active agent must possess core element of secrecy (4)

I believe the answer is:

spry

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'active' is the definition.
(similar in meaning)

'possess core element of secrecy' is the wordplay.
'core' says to take the centre.
'element of secrecy' becomes 'pry' (I am not sure about this - if you are sure you should believe this answer much more).
The middle letter of 'possess' is 's'.
's'+'pry'='SPRY'

'agent must' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.

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(Other definitions for spry that I've seen before include "Energetic" , "Lively (especially of an older person)" , "Sprightly" , "Quick and agile" , "Nimble, brisk" .)

I've seen this clue in The Telegraph.

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