Shout was one of hers coming back: worried, heading to scary howls (8)
I believe the answer is:
'howls' is the definition.
('ululate' can be a synonym of 'howl')
'shout was one of hers coming back worried heading to scary' is the wordplay.
'shout was one of hers' becomes 'lulu' (Shout is a song by 60's singer Lulu).
'coming back' shows that the letters should be reversed in order.
'worried' becomes 'ate' (something worrying may eat someone up).
'heading to' says to take the initial letters.
The initial letter of 'scary' is 's'.
'lulu' backwards is 'ulul'.
(Other definitions for ululates that I've seen before include "Howls, wails" , "Emits long loud cries" .)