Sound behaviour provides freedom of movement (4,7)

I believe the answer is:

safe conduct

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'freedom of movement' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'sound behaviour' is the wordplay.
'sound' becomes 'safe' (similar in meaning).
'behaviour' becomes 'conduct' (synonyms).
'safe'+'conduct'='SAFE-CONDUCT'

'provides' acts as a link.

Can you help me to learn more?

(Another definition for safe conduct that I've seen is " Immunity from arrest when travelling".)

I've seen this clue in The Daily Express.

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