Fruitless undertaking (5,6)
I believe the answer is:
fools errand
'fruitless undertaking' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fools errand that I've seen before include "Wild goose chase" , "Fruitless enterprise" , "Unsuccessful enterprise" , "Fruitless undertaking" , "Fruitless task" .)