Senator one recalled, an adviser to Nero (6)
I believe the answer is:
seneca
'an adviser to nero' is the definition.
(I've seen this in another clue)
'senator one recalled' is the wordplay.
'senator' becomes 'sen'.
'one' becomes 'ace' (ace has a value of one in some card games).
'recalled' shows that the letters should be reversed in order (to recall can mean to bring something back).
'ace' in reverse letter order is 'eca'.
'sen'+'eca'='SENECA'
(Other definitions for seneca that I've seen before include "Encase the Roman statesman and playwright badly" , "Roman philosopher; North American Indian" , "Roman statesman, philosopher and playwright" , "philosopher in Rome" , "Dramatist" .)