Crossword Genius

Scowl down upon a gangster leader (6)

Ross

I believe the answer is:

glower

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

'scowl' is the definition.
(I've seen this before)

'down upon a gangster leader' is the wordplay.
I cannot quite understand how this works, but
'down' could be 'lower' (synonyms) and 'lower' is located in the answer.
A single letter 'g' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for glower that I've seen before include "firefly?" , "Frown" , "Angry and sullen look" , "shiner?" , "Threatening look" .)

I've seen this clue in the Evening Standard.
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