A little one to many (4)
I believe the answer is:
atom
'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 "Smallest particle in an element able to take part in a chemical reaction" , "A mighty particle, can be explosive" , "The tiniest particle" , "Small amount of matter" , "A tiny particle of matter with a big bang" .)