Assistant promoting last part of plan (4)
I believe the answer is:
aide
'assistant' is the definition.
(aide is a kind of assistant)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for aide that I've seen before include "Right-hand man/woman" , "Second" , "Adviser or assistant" , "auxiliary" , "Assistant to a more senior person" .)