The most common and most likely answer to this crossword clue is the 7 letter word TOPMOST
I've seen this crossword clue in The Daily Telegraph
T
O
P
M
O
S
T
Buddy explains!

The straight part
We know that "Peak" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case peak = topmost.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 TOPMOST mean?
TOPMOST adjective- At or nearest to the top; uppermost; being the very highest.