America's new topless messengers (8)
I believe the answer is:
apostles
'messengers' is the definition.
'america's new topless' is the wordplay.
'america' becomes 'a' (common abbreviation - e.g. in organisation names).
'new' indicates anagramming the letters.
'topless' anagrammed gives 'postles'.
'a'+'postles'='APOSTLES'
(Other definitions for apostles that I've seen before include "Disciples chosen by Jesus" , "Twelve disciples chosen by Christ" , "They were sent by Christ to teach the Gospel" , "Evangelists" , "Early Christians" .)