Crossword Genius

Undermine one's boss but give no indication (5,4,4)

Ross

I believe the answer is:

shake your head

I'm a little stuck... Click here to teach me more about this clue!

'undermine' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'one's boss but give no indication' is the wordplay.
I cannot quite see how this works, but
'boss' could be 'head' (synonyms) and 'head' is found within the answer.
'no' could be 'o' (O resembles 0) and 'o' is found within the answer.
'indication' could be 'key' (I've seen this in another clue) and 'key' is located in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Globe and Mail.
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