The most common and most likely answer to this crossword clue is the 5 letter word BAIRN
I've seen this crossword clue in The Guardian
B
A
I
R
N
More clues leading to the result BAIRN
- Mac, small one I found in store
- Child, one imprisoned in outbuilding
- Large empty building houses one Scottish child
- Wee one, born one of a set in Carnoustie?
- Irish veto banking issue in Glasgow
- Child I located in outbuilding
- One sitting in outbuilding in Scotland, 21
- Outbuilding accommodating single wee Scot?
- Ex-politician, without his socialist label, heads north to create youngster
- Child and I must go into farm building
Buddy explains!

The straight part
We know that "Mac, small one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mac, small one = bairn.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.
What does BAIRN mean?
BAIRN noun- A child or baby.