The most common and most likely answer to this crossword clue is the 5 letter word DOGGO
I've seen this crossword clue in The Guardian
D
O
G
G
O
More clues leading to the result DOGGO
- Remaining quietly hidden takes trouble and effort
- Leave after pet is safely hidden
- Leave after trouble, so as not to be seen
- How one may lie in party game, stutteringly
- Hidden plague, get lost!
- Still in party? Slip away after girl’s left
- Mum not seen to follow 15
- Still one way to lie
- Out of sight, dead horse is consumed by ducks
- How Brer Fox lay? Leave after him?
Buddy explains!

The straight part
We know that "Mum not seen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mum not seen = doggo.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 DOGGO mean?
DOGGO noun- A dog.