Old statesman exercised coercion on leaving (6)
I believe the answer is:
cicero
'old statesman' is the definition.
(Roman politician)
'exercised coercion on leaving' is the wordplay.
'exercised' is an anagram indicator.
'leaving' is a deletion indicator.
'coercion' with 'on' removed is 'coerci'.
'coerci' is an anagram of 'CICERO'.
(Other definitions for cicero that I've seen before include "Old Roman statesman and orator" , "Statesman from Rome" , "Philosopher" , "Orator's" , "One-time consul" .)