The very old friend of Pytheas, one may imagine (5)
I believe the answer is:
damon
'the very old friend' is the definition.
'damon' can be an answer for 'friend' (I have seen 'faithful friend' mean 'damon' so perhaps 'friend' could also mean 'damon'). I'm not sure about the 'the very old' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for damon that I've seen before include "faithful friend" , "Matt --, US actor" , "Writer Runyon's first name" , "Pythias's friend" .)