Sockdolager (4)
I believe the answer is:
lulu
'sockdolager' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lulu that I've seen before include "Berg opera; outstanding thing (colloq.)" , "Berg's unfinished opera" , "Outstanding person or thing" , "Outstanding person; singer" , "UK 14 7 [EURO VISION] entrant" .)