No longer forward, nevertheless advanced (6)
I believe the answer is:
expert
'nevertheless advanced' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives can occasionally mean the same thing.
'no longer forward' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'forward' becomes 'pert' (both can mean cheeky or disrespectful).
'ex'+'pert'='EXPERT'
Can you help me to learn more?
(Other definitions for expert that I've seen before include "At the right moment" , "Pro" , "Proficient and skilful" , "Skilled workman or specialist" , "Dab hand" .)