Predictable joint over silly guy (8)

I believe the answer is:

kneejerk

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'predictable' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'joint over silly guy' is the wordplay.
'joint' becomes 'knee' (knee joint is a kind of joint).
'over silly guy' becomes 'jerk' (I am not sure about this - if you are sure you should believe this answer much more).
'knee'+'jerk'='KNEEJERK'

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I've seen this clue in the Sydney Morning Herald.

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