Orator curtailed boast, retaining reserve (6)
I believe the answer is:
Here is my best explanation:
'orator' is the definition.
(I've seen this before)
'curtailed boast retaining reserve' is the subsidiary indication.
'curtailed' means to remove the last letter.
'boast' becomes 'crow' (crow is a kind of boast).
'retaining' is an insertion indicator.
'reserve' becomes 'ice' (I've seen this before).
'crow' with its last letter removed is 'cro'.
'cro' going around 'ice' is 'cicero'.
(Other definitions for cicero that I've seen before include "Orator's", "Roman stylist, orator", "Old Roman statesman and orator", "Famous Roman orator", "Roman orator, author", "Roman lawyer", "Philosopher", "accomplished orator", "Orator" and "wartime spy".)