The most common and most likely answer to this crossword clue is the 7 letter word PYJAMAS
I've seen this crossword clue in The Guardian
P
Y
J
A
M
A
S
More clues leading to the result PYJAMAS
- . Extremely pretty stuff like dark clothing?
- Dress for retirement party outside with jacket top when worn by old lady
- Threads for crashing out
- Case of puppy, 3, as the best thing with the cat?
- Riser rising around end of day, then trouble set for the night?
- Casual habits causing leader in police force to break state up
- Sick pay’s retained to preserve what one’s invested in for retirement?
- They took over from the night-shift
- My asp slithered around Japanese Australian’s nightdress
- First of all, probing young journalist asked me about strange bedtime habit?
Buddy explains!

The straight part
We know that "the best thing with the cat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the best thing with the cat = pyjamas.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 PYJAMAS mean?
PYJAMAS noun- Clothes for wearing to bed and sleeping in, usually consisting of a loose-fitting shirt and pants/trousers.
- Loose-fitting trousers worn by both sexes in various southern Asian countries including India.