Nucleus of bigger machine (4)
I believe the answer is:
germ
'nucleus' is the definition.
(thesaurus)
'of bigger machine' is the wordplay.
'of' indicates a hidden word.
'GERM' is hidden in the letters of 'bigger machine'.
(Other definitions for germ that I've seen before include "Cause of some diseases" , "Microorganism causing disease" , "starting point" , "Bug" , "virus?" .)