Smile and leave American to Mexican (6)
I believe the answer is:
gringo
'american to mexican' is the definition.
(Spanish term for someone who is not Hispanic)
'smile and leave' is the wordplay.
'smile' becomes 'grin' (grinning is a kind of smiling).
'and' says to put letters next to each other.
'leave' becomes 'go'.
'grin'+'go'='GRINGO'
(Other definitions for gringo that I've seen before include "Latin-American term for English speaker" , "Non-Spanish speaker" , "What the Mexican calls the American" , "English-speaker abroad" .)