A complex plot's beginning to engross Peter perhaps (7)
I believe the answer is:
apostle
'peter perhaps' is the definition.
(peter is a kind of Apostle)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for apostle that I've seen before include "Divine messenger" , "Holy person" , "Peter, perhaps" , "Disciple of Jesus Christ" , "Ale pots (anag)" .)