The most common and most likely answer to this crossword clue is the 7 letter word NIPPIER
I've seen this crossword clue in Financial Times
N
I
P
P
I
E
R
Buddy explains!

The straight part
We know that "less warm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case less warm = nippier.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 NIPPIER mean?
NIPPIER adjective- Fast; speedy
- Of the weather, rather cold.
- Inclined to nip; bitey.
- Annoying; irritating
- Sharp in taste
- Curt
- Parsimonious