The most common and most likely answer to this crossword clue is the 4 letter word JOEY
I've seen this crossword clue in The Times
J
O
E
Y
Buddy explains!

The straight part
We know that "kangaroo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case kangaroo = joey.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 JOEY mean?
JOEY noun- The immature young of a marsupial, notably a junior kangaroo, but also a young wallaby, koala, etc.
- The shorter word whose letters can be found within a kangaroo word.
- A parcel smuggled in to an inmate.
- (circus) A kind of clown.
- A member of the Royal Marines.
- A person with cerebal palsy.
- A stupid person.