One on board often depressed: restraint vital (7,3)

I believe the answer is:

control key

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'one on board often depressed' is the definition.
I can't judge whether this defines the answer.

'restraint vital' is the wordplay.
'restraint' becomes 'control' (synonyms).
'vital' becomes 'key' (key can mean important).
'control'+'key'='CONTROL KEY'

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I've seen this clue in The Times.

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