Old Roman spies given new name under cover (5)
I believe the answer is:
cinna
'old roman' is the definition.
(Roman politician)
'spies given new name under cover' is the wordplay.
'spies' becomes 'cia'.
'given' means one lot of letters goes inside another.
'new' becomes 'n'.
'name' becomes 'n' (common abbreviation e.g. on forms).
'under cover' means one lot of letters goes inside another.
'cia' going around 'n' is 'cina'.
'cina' enclosing 'n' is 'CINNA'.
(Other definitions for cinna that I've seen before include "conspirator in Rome" , "Conspirator against Caesar" , "Poet" , "conspirator and wicked type, some say" , "Roman consul" .)