Relax when on the witness box? No problem! (5,4)

I believe the answer is:

stand easy

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'relax' is the definition.
(thesaurus)

'on the witness box? no problem' is the wordplay.
'on' means one lot of letters go next to another.
'the witness box?' becomes 'stand' (I can't justify this - if you can you should believe this answer much more).
'no problem' becomes 'easy' (I've seen this in other clues).
'stand'+'easy' is 'STAND EASY'.

'when' acts as a link.

Can you help me to learn more?

(Another definition for stand easy that I've seen is " Soldiers thus ordered".)

I've seen this clue in the Evening Standard.

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