Scowl down upon a gangster leader (6)
I believe the answer is:
glower
'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" .)