Set up arrest as practical joke (3-4)
I believe the answer is:
leg-pull
'practical joke' is the definition.
(I've seen this before)
'set up arrest' is the wordplay.
'set' becomes 'gel' (both can mean to harden).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'arrest' becomes 'pull' (I can't explain this - if you can you should give a lot more credence to this answer).
'gel' reversed gives 'leg'.
'leg'+'pull'='LEG-PULL'
'as' is the link.
Can you help me to learn more?
(Other definitions for leg-pull that I've seen before include "Trick; practical joke" , "Jolly jape involving lower limb" , "Trying to make somebody believe what is not true as a joke" , "Attempt, as a joke, to make somebody believe something that is not true" , "A hoax, tease" .)