Go on holiday? You must be joking! (3,4)
I believe the answer is:
get away
'go on holiday? you must be joking' is the definition.
I can't tell whether this definition defines the answer.
'go on holiday?' is the wordplay.
'go' becomes 'get' (synonyms).
'on holiday?' becomes 'away' (I've seen this before).
'get'+'away'='GET-AWAY'
Can you help me to learn more?
(Other definitions for get away that I've seen before include "Escape; short holiday" , "Breakout" , "take a holiday" , "Escape; come off it!" , "The car that provides escape for criminal" .)