Ad lib is put in to make enhancement (9)
I believe the answer is:
improvise
'ad lib' is the definition.
(thesaurus)
'is put in to make enhancement' is the wordplay.
'put in' is an insertion indicator.
'to make enhancement' becomes 'improve'.
'is' placed inside 'improve' is 'IMPROVISE'.
(Other definitions for improvise that I've seen before include "wing it" , "Talk without notes" , "Vamp" , "Perform without preparation" , "Act without previous planning" .)