Professional comes in to introduce new generation (12)
I believe the answer is:
reproduction
'generation' is the definition.
(thesaurus)
'professional comes in to introduce new' is the wordplay.
'professional' becomes 'pro' ('pro' can be a synonym of 'professional').
'comes in to' is an insertion indicator.
'new' indicates an anagram.
'introduce' anagrammed gives 'reduction'.
'pro' inserted into 'reduction' is 'REPRODUCTION'.
(Other definitions for reproduction that I've seen before include "sound quality" , "Print made of a painting" , "Copy (of a work of art)" , "Picture" , "Copy of an artwork" .)