He joins a biblical character - an old slave (5)
I believe the answer is:
helot
'an old slave' is the definition.
'helot' can be an answer for 'slave' (slave in ancient Sparta). I'm not certain of the 'an old' bit.
'he joins a biblical character' is the wordplay.
'joins' is a charade indicator (letters next to each other).
'a biblical character' becomes 'lot' (I've seen this in another clue).
'he'+'lot'='HELOT'
Can you help me to learn more?
(Other definitions for helot that I've seen before include "downtrodden?" , "He's bound to land" , "Spartan slave" , "Hotel (anag) - serf" , "One made to work" .)