An old enemy (4)
I believe the answer is:
boer
'enemy' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'an old' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
The remaining letters 'ber' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for boer that I've seen before include "Afrikaner" , "Dutch settler in southern Africa" , "One we fought in 1900" , "South African of Dutch ancestry" , "Robe worn by S African rebel" .)