The most common and most likely answer to this crossword clue is the 7 letter word SUMMERY
I've seen this crossword clue in The Guardian
S
U
M
M
E
R
Y
Buddy explains!

The straight part
We know that "Warm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case warm = summery.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 SUMMERY mean?
SUMMERY adjective- Relating to the summer.
- Of weather, typical of summer.