As a joke, giving a little session on the rack to? (3-4)
I believe the answer is:
leg-pull
'as' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for leg-pull that I've seen before include "Informal hoax involving lower limb" , "Trick; practical joke" , "Attempt, as a joke, to make somebody believe something that is not true" , "A hoax, tease" , "Jolly jape involving lower limb" .)