Tough, glum face hidden under American's bonnet (7)
I believe the answer is:
hoodlum
'tough' is the definition.
(I know that tough can be written as hoodlum)
'glum face hidden under american's bonnet' is the wordplay.
'face hidden' means to remove the first letter (the front or 'face' of the word is removed).
'under' says to put letters next to each other (below, in a down clue).
'american's bonnet' becomes 'hood' (US term for a car bonnet).
'glum' with its first letter removed is 'lum'.
'lum' after 'hood' is 'HOODLUM'.
(Other definitions for hoodlum that I've seen before include "Petty gangster" , "Young thug or gangster" , "Lawless youth" , "Young thug or criminal" , "Aggressive young criminal" .)