Crossword Genius

A little one to many (4)

Ross

I believe the answer is:

'a little' is the definition.
(thesaurus)

'one to many' is the wordplay.
'one' becomes 'a'.
'many' becomes 'm' ().
'a'+'to'+'m'='ATOM'

(Other definitions for atom that I've seen before include "ounce", "Smallest particle in an element able to take part in a chemical reaction", "A small but mighty particle", "Smallest amount", "A wee bit".)