A sign of more to follow (4)

I believe the answer is:

plus

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

'a sign of more' is the definition.
'plus' can be an answer for 'more' (I've seen this before). I'm not sure about the 'a sign of' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for plus that I've seen before include "To the French it's more" , "An advantage (colloq.)" , "And also; a bonus" , "An advantage; positive" , "Together with; advantage" .)

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

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