Said to shut it before exam, though there's a passage of text with some words missing (5,4)

'said to shut it' is the definition.
I can't tell whether this definition defines the answer.

'exam though there's a passage of text with some words missing' is the wordplay.
I cannot really understand how this works, but
'exam' could be 'test' (synonyms) and 'test' is found in the answer.
'of' could be 'o' and 'o' is present in the answer.
This explanation may well be incorrect...

'before' acts as a link.

Can you help me to learn more?