Old Scots about to enter mines (5)
I believe the answer is:
picts
'old scots' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'about to enter mines' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to enter' is an insertion indicator.
'mines' becomes 'pits' (I've seen this in another clue).
'c' inserted within 'pits' is 'PICTS'.
Can you help me to learn more?
(Other definitions for picts that I've seen before include "Ancient Northerners" , "People living in Scotland in Roman times" .)