Sweets go astray, what's more, that one's sent on in vain (5,6)
I believe the answer is:
fools errand
'one's sent on in vain' is the definition.
I can't judge whether this defines the answer.
'sweets go astray what's more' is the wordplay.
'sweets' becomes 'fools' (I can't justify this - if you can you should give a lot more credence to this answer).
'go astray' becomes 'err' (to err is to make an error or go the wrong way).
'what's more' becomes 'and' (I've seen this in another clue).
'fools'+'err'+'and'='FOOLS ERRAND'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for fools errand that I've seen before include "Wild goose chase" , "Fruitless undertaking" , "Hopeless task" , "Fruitless enterprise" , "Unsuccessful enterprise" .)