Ad lib, as rascal is over-excited (9)
I believe the answer is:
improvise
'ad lib' is the definition.
(I've seen this before)
'rascal is over-excited' is the wordplay.
'rascal' becomes 'imp' (synonyms).
'excited' indicates anagramming the letters.
'is'+'over'='isover'
'isover' with letters rearranged gives 'rovise'.
'imp'+'rovise'='IMPROVISE'
'as' acts as a link.
(Other definitions for improvise that I've seen before include "Vamp" , "Create without preparation" , "ad lib" , "wing it" , "Extemporise" .)