Again settled up American youngster's requirement (6)
I believe the answer is:
diaper
'american youngster's requirement' is the definition.
(US term for nappy)
'again settled up' is the wordplay.
'again settled' becomes 'repaid' ('paid' is an answer for 'settled' so perhaps 'repaid' is OK for 'again settled').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'repaid' written backwards gives 'DIAPER'.
(Other definitions for diaper that I've seen before include "An American nappy" , "type of paving" , "Baby's nappy" .)