Affirm that a cricket fan will be there (6)
I believe the answer is:
attest
'affirm' is the definition.
(I know that attest is a more specific form of the action affirm)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Give sworn evidence" , "Give evidence in a court of law" , "Affirm by oath" , "Give testimony in a court of law" , "Sign on" .)