Extremely resolute one-time agent of change (8)
I believe the answer is:
reformer
'agent of change' is the definition.
(someone responsible for change or reform)
'extremely resolute one-time' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'one-time' becomes 'former' (I've seen this in another clue).
'resolute' with its middle taken out is 're'.
're'+'former'='REFORMER'
(Other definitions for reformer that I've seen before include "One changing for the better" , "campaigner for change" , "Luther, perhaps" , "One making improving changes" , "One who changes or improves, say, an organisation" .)