User began to change smaller groups (9)
I believe the answer is:
subgenera
'smaller groups' is the definition.
The definition suggests a plural noun which matches the answer.
'user began to change' is the wordplay.
'to change' indicates anagramming the letters.
'user'+'began'='userbegan'
'userbegan' with letters rearranged gives 'SUBGENERA'.
Can you help me to learn more?
(Other definitions for subgenera that I've seen before include "Small groups" , "taxonomic groups" , "Biological groups" , "Biology classes" .)