Niggle (4)
I believe the answer is:
fuss
'niggle' is the definition.
(I know that niggle can be written as fuss)
This is the entire clue.
(Other definitions for fuss that I've seen before include "Protest, complaint" , "Bustle and bother" , "To-do" , "Unnecessary agitation" , "Bustle; ado" .)