Become a candidate for a professional examination, but put in an objection (5,1,7)

I believe the answer is:

enter a protest

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'become a candidate for a professional examination' is the definition.
I can't judge whether this defines the answer.

'put in an objection' is the wordplay.
'put in' becomes 'enter' (I've seen this in another clue).
'an' becomes 'a'.
'objection' becomes 'protest' (synonyms).
'enter'+'a'+'protest'='ENTER A PROTEST'

'but' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Independent.

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