One who can bridle at Americans somewhat (5)
I believe the answer is:
tamer
'one who can bridle' is the definition.
(someone who can tame or bridle a horse)
'at americans somewhat' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'TAMER' is hidden within 'at americans'.
(Other definitions for tamer that I've seen before include "less offensive?" , "Less arousing, exciting" , "Less wild" , "Animal trainer" .)