The most common and most likely answer to this crossword clue is the 8 letter word LAMBSKIN
I've seen this crossword clue in The Times
L
A
M
B
S
K
I
N
More clues leading to the result LAMBSKIN
Buddy explains!

The straight part
We know that "warmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case warmer = lambskin.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 LAMBSKIN mean?
LAMBSKIN noun- The skin of a very young sheep, especially prepared with the fleece still attached. A very fine form of leather.
- A kind of woollen cloth resembling this.