Serious offence requires time to think (7)
I believe the answer is:
treason
'serious offence' is the definition.
'treason' can be an answer for 'offence' (I've seen this before). I am unsure of the 'serious' bit.
'time to think' is the wordplay.
'time' becomes 't' (abbreviation).
'to think' becomes 'reason' (reasoning is a kind of thinking).
't'+'reason'='TREASON'
'requires' is the link.
Can you help me to learn more?
(Other definitions for treason that I've seen before include "attempting to overthrow the king" , "Crime of treachery against the state" , "Act of betrayal of a government" , "Lese-majesty" , "Crime agains one's country" .)