Department head is driven to protest (7)

I believe the answer is:

dissent

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'to protest' is the definition.
(synonyms)

'department head is driven' is the wordplay.
'head' says to take the initial letters.
'driven' becomes 'sent' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'department' is 'd'.
'd'+'is'+'sent'='DISSENT'

Can you help me to learn more?

(Other definitions for dissent that I've seen before include "Raise objections" , "Differ" , "Disagree" , "Dispute" , "Not agree" .)

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

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