The most common and most likely answer to this crossword clue is the 5 plus 1 letter word SNUG –
S
N
U
G
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“
Buddy explains!

The straight part
We know that "Warm and/ cosy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case warm and/ cosy = snug –.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 SNUG – mean?
SNUG – noun- A small, comfortable back room in a pub.
- A lug.
- To make secure or snug.
- To snuggle or nestle.
- To make smooth.
- Warm and comfortable; cosy.
- Satisfactory.
- Close-fitting.
- Close; concealed; not exposed to notice.