The most common and most likely answer to this crossword clue is the 9 letter word BALACLAVA
B
A
L
A
C
L
A
V
A
More clues leading to the result BALACLAVA
- Cold volcanic rock underlying Welsh lake becomes warmer?
- Bill stopping dance to find girl’s hat
- Airman, in poem, endlessly against ace in battle
- Bill and John in Welsh lake needing head protection
- Battle mask
- Battle over a party with Corbyn’s lead — it’s very hot!
- Battle helmet
- You wouldn’t keep a cool head in this battle
- Hood’s battle
- Apt headgear for a Victorian engagement?
Buddy explains!

The straight part
We know that "head warmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case head warmer = balaclava.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 BALACLAVA mean?
BALACLAVA noun- A warm cap shielding the neck and head, often made out of wool.
- A ski mask with holes for the eyes and, sometimes, the nose and mouth, which may be rolled up and worn like a toque or pulled over the face for greater protection.