Lively serenade about faction (8)
I believe the answer is:
swinging
'lively' is the definition.
(I've seen this before)
'serenade about faction' is the wordplay.
'serenade' becomes 'sing' (to serenade someone is to sing to them).
'about' is an insertion indicator.
'faction' becomes 'wing' (a group within an organisation).
'sing' enclosing 'wing' is 'SWINGING'.
(Other definitions for swinging that I've seen before include "Hung and moving to and fro" , "Lively, fashionable" , "The Sixties were said to be this" , "Moving back and forth" , "Vacillating" .)