Standard gen's in for deciphering (6)

I believe the answer is:

ensign

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'standard' is the definition.
(both can mean a flag)

'gen's in for deciphering' is the wordplay.
'gen's in' can be anagrammed to 'ENSIGN'.
Though, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for ensign that I've seen before include "Badge, flag" , "Small flag or junior naval officer" , "Ship's colours" , "Flag - officer, formerly" , "Standard - flag" .)

I've seen this clue in The Sun.

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