Suitable expression for the sergeant major giving an order? (5,4)

I believe the answer is:

right face

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'the sergeant major giving an order?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'suitable expression' is the wordplay.
'suitable' becomes 'right' (synonyms).
'expression' becomes 'face' (I can't justify this - if you can you should give a lot more credence to this answer).
'right'+'face'='RIGHT FACE'

'for' acts as a link.

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