Farewell sign of a lieutenant? (3,3)

I believe the answer is:

pip pip

'farewell' is the definition.
(phrase used to say goodbye)

'sign of a lieutenant?' is the wordplay.
I cannot quite see how this works, but
'sign' could be 'pi' (pi is a kind of sign) and 'pi' is present in the answer.
This explanation may well be incorrect...

I've seen this clue in The Times.

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