Silly twit in charge is beginning to make a joke (9)
I believe the answer is:
witticism
'a joke' is the definition.
(thesaurus)
'silly twit in charge is beginning to make' is the wordplay.
'silly' is an anagram indicator.
'in charge' becomes 'ic' (abbreviation for 'in charge').
'beginning to' suggests taking the first letters.
The first letter of 'make' is 'm'.
'twit' is an anagram of 'witt'.
'witt'+'ic'+'is'+'m'='WITTICISM'
(Other definitions for witticism that I've seen before include "Clever remark" , "Sparkling remark" , "Quip" .)