Orator curtailed boast, retaining reserve (6)
I believe the answer is:
cicero
'orator' is the definition.
(I've seen this before)
'curtailed boast retaining reserve' is the wordplay.
'curtailed' means to remove the last letter.
'boast' becomes 'crow' (crow is a kind of boast).
'retaining' indicates putting letters inside.
'reserve' becomes 'ice' (I've seen this before).
'crow' with its final letter taken off is 'cro'.
'cro' going around 'ice' is 'CICERO'.
(Other definitions for cicero that I've seen before include "Roman orator and master of prose" , "wartime spy" , "Philosopher" , "Orator's" , "Roman lawyer" .)