French setter will flounce (5)
I believe the answer is:
frill
'flounce' is the definition.
(I know that flounce can be written as frill)
'french setter will' is the wordplay.
'french' becomes 'fr' (abbreviation).
'setter will' becomes 'ill' (I'll, i.e. 'the person writing the crossword will').
'fr'+'ill'='FRILL'
(Other definitions for frill that I've seen before include "Pleated strip of cloth trimming or an unnecessary extra" , "Tuck" , "Maybe superfluous piece of adornment" , "Unnecessary embellishment" , "Extra embellishment" .)