They're a blend of the main races (9)
I believe the answer is:
americans
'they're a blend' is the definition.
The definition suggests an adverb but the answer is not.
'of the main races' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'main'+'races'='mainraces'
'mainraces' with letters rearranged gives 'AMERICANS'.
Can you help me to learn more?
(Other definitions for americans that I've seen before include "New Yorkers, e.g." , "Rains came (anag.)" , "Yanks" , "across the pond?" .)