Illustrious companion, a pilot, runs off (8)
I believe the answer is:
palatial
'illustrious' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'companion a pilot runs off' is the wordplay.
'companion' becomes 'pal' (I've seen this in another clue).
'pilot runs off' becomes 'tial' (I can't explain this - if you can you should give a lot more credence to this answer).
'pal'+'a'+'tial'='PALATIAL'
Can you help me to learn more?
(Other definitions for palatial that I've seen before include "Stately, imposing" , "Hugely impressive" , "Magnificently appointed" , "(Of building) spacious and grand" , "Sumptuous, like a royal dwelling" .)