Knocked over, Greek character and singer howl (7)
I believe the answer is:
ululate
'howl' is the definition.
('ululate' can be a synonym of 'howl')
'knocked over greek character and singer' is the wordplay.
'knocked over' says the letters should be written in reverse.
'greek character' becomes 'eta' (letter from the Greek alphabet).
'and' is a charade indicator (letters next to each other).
'singer' becomes 'lulu' (I've seen this before**).
'eta'+'lulu'='etalulu'
'etalulu' backwards is 'ULULATE'.
(Other definitions for ululate that I've seen before include "Howl, wail, screech" , "Wail in grief" , "Give a hoot" , "Howl or wail as in mourning" , "Howl loudly" .)