Reactor of no greater form (9)
I believe the answer is:
generator
'reactor' is the definition.
(thesaurus)
'of no greater form' is the wordplay.
'of' becomes 'o''.
'no' becomes 'n' (common abbreviation).
'form' indicates an anagram (a different form of the same letters).
'o'+'n'+'greater'='ongreater'
'ongreater' with letters rearranged gives 'GENERATOR'.
(Other definitions for generator that I've seen before include "Electricity producer" , "Device creating electrical energy" , "Existing in name only" , "It gives power" , "juice maker" .)