Child’s first with yen for American sweets (5)
I believe the answer is:
candy
'american sweets' is the definition.
(I've seen this before)
'child's first with yen for' is the wordplay.
'child's first' becomes 'c' (1st letter of 'child').
'with' becomes 'and'.
'yen for' becomes 'y' (I've seen this in other clues).
'c'+'and'+'y'='CANDY'
(Other definitions for candy that I've seen before include "American confectionary" , "Girl's name (diminutive)" , "Sugary sweet" , "Crystallised sugar; US sweets" , "Sweet (US)" .)