Humble American on foot (5)
I believe the answer is:
abase
'humble' is the definition.
(synonyms)
'american on foot' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'on' says to put letters next to each other.
'foot' becomes 'base' (synonyms).
'a'+'base'='ABASE'
(Other definitions for abase that I've seen before include "Cast down" , "Belittle, degrade" , "what 11 [REPRIMANDS] are meant to do" , "put to shame" , "Humble (oneself)" .)