The most common and most likely answer to this crossword clue is the 6 letter word OKAPIS
I've seen this crossword clue in The Guardian
O
K
A
P
I
S
Buddy explains!

The straight part
We know that "Giraffes’ cousins" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case giraffes’ cousins = okapis.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 OKAPIS mean?
OKAPIS noun- A large ruminant mammal, Okapia johnstoni, found in the rainforests of the Congo, related to the giraffe, but with a much shorter neck, a reddish brown coat and zebra-like stripes on its hindquarters.