The most common and most likely answer to this crossword clue is the 12 letter word PROMONTORIES
I've seen this crossword clue in The Sunday Telegraph
P
R
O
M
O
N
T
O
R
I
E
S
More clues leading to the result PROMONTORIES
Buddy explains!

The straight part
We know that "Capes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case capes = promontories.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.
Crossword Answer definitions
PROMONTORIES noun- A high point of land extending into a body of water, headland; cliff.
- A projecting part of the body.