Holidays cancelled? You're joking (5,3)
I believe the answer is:
leave off
'you're joking' is the definition.
I can't judge whether this defines the answer.
'holidays cancelled?' is the wordplay.
'holidays' becomes 'leave'.
'cancelled?' becomes 'off' ('off' can be similar in meaning to 'cancelled').
'leave'+'off'='LEAVE OFF'
Can you help me to learn more?
(Other definitions for leave off that I've seen before include "not eating or drinking" , "Cease, or shed garment" , "Give over!" .)