Pressure applied to idiot: "I don't know the answer" (4)
I believe the answer is:
pass
'i don't know the answer' is the definition.
(you might say 'pass' to skip a question)
'pressure applied to idiot' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'applied to' means one lot of letters go next to another.
'idiot' becomes 'ass' (both can mean a stupid person).
'p'+'ass'='PASS'
(Other definitions for pass that I've seen before include "Go by; succeed in (an exam)" , "complimentary ticket" , "Progress" , "Don't answer a question" , "Exceed; leave behind" .)