Cancel summons seeing upper-class type swallow drug (5,3)
I believe the answer is:
write off
'cancel' is the definition.
(writing off is a kind of cancelling)
'summons seeing upper-class type swallow drug' is the wordplay.
'summons' becomes 'writ' (I've seen this before).
'seeing upper class type' becomes 'off' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'swallow' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'writ'+'off'='writoff'
'writoff' placed around 'e' is 'WRITE-OFF'.
Can you help me to learn more?
(Other definitions for write off that I've seen before include "Contact the insurance company, perhaps" , "Adjustment made by bookkeeper" , "Ruined car" , "Post application" , "trash" .)