Smaller groups shuffling by push large American (8)
I believe the answer is:
subphyla
'smaller groups' is the definition.
I know nothing about this answer so I can't judge whether this works.
'shuffling by push large american' is the wordplay.
'shuffling' indicates an anagram.
'large' becomes 'L' (eg in clothes sizes).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'by'+'push'='bypush'
'bypush' anagrammed gives 'subphy'.
'subphy'+'l'+'a'='SUBPHYLA'
Can you help me to learn more?
(Another definition for subphyla that I've seen is " Zoological taxonomic ranks immediately above 'superclass'".)