Round out UN with Pole (3)
I believe the answer is:
rod
'pole' is the definition.
(synonyms)
'round out un' is the wordplay.
'out' indicates named letters should be taken away.
'round' with 'un' taken away is 'ROD'.
'with' acts as a link.
(Other definitions for rod that I've seen before include "Old saying 'Spare the . . . and spoil the child'" , "Baton, pole" , "Long, straight stick" , "Cane - shaft" , "Fishing implement" .)