Sweet American sent back by boy (6)
I believe the answer is:
sugary
'sweet' is the definition.
(I've seen this before)
'american sent back by boy' is the wordplay.
'american' becomes 'us'.
'sent back' says the letters should be written in reverse.
'by' says to put letters next to each other.
'boy' becomes 'gary' (**).
'us' back-to-front is 'su'.
'su'+'gary'='SUGARY'
(Other definitions for sugary that I've seen before include "Excessively sentimental" , "Excessively sweet or saccharine" , "Sweet, syrupy" , "Sweet, perhaps too much so" , "Over-sentimental" .)