The most common and most likely answer to this crossword clue is the 10 letter word UNFRIENDLY
U
N
F
R
I
E
N
D
L
Y
Buddy explains!

The straight part
We know that "Frosty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case frosty = unfriendly.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 UNFRIENDLY mean?
UNFRIENDLY noun- An enemy.
- Not friendly; hostile; mean.
- Unfavourable.
- In an unkind or unfriendly manner; not as a friend