Funny guy's agreement secured by another (8)

'funny guy's' is the definition.
(someone who makes jokes)

'agreement secured by another' is the wordplay.
'agreement' becomes 'ok' (I've seen this in other clues).
'secured by' is an insertion indicator (inserted letters are held or secured).
'another' becomes 'jester' (I can't explain this - if you can you should give a lot more credence to this answer).
'ok' inserted into 'jester' is 'JOKESTER'.