The most common and most likely answer to this crossword clue is the 7 plus 5 letter word BELGIAN CONGO
I've seen this crossword clue in The Times
B
E
L
G
I
A
N
C
O
N
G
O
Buddy explains!
The straight part
We know that "old part of Africa" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old part of africa = belgian congo.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.