The most common and most likely answer to this crossword clue is the 5 plus 6 letter word SOUTH AFRICA
S
O
U
T
H
A
F
R
I
C
A
More clues leading to the result SOUTH AFRICA
Buddy explains!
The straight part
We know that "Commonwealth nation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case commonwealth nation = south africa.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.