The most common and most likely answer to this crossword clue is the 5 letter word HOBBY
I've seen this crossword clue in The Daily Telegraph
H
O
B
B
Y
More clues leading to the result HOBBY
Buddy explains!
The straight part
We know that "nag in evidence when child is playing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nag in evidence when child is playing = hobby.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 HOBBY mean?
HOBBY noun- An activity that one enjoys doing in one's spare time.
- An extinct breed of horse native to the British Isles, also known as the Irish Hobby
- Any of four species of small falcons in the genus Falco, especially Falco subbuteo.