The most common and most likely answer to this crossword clue is the 11 letter word SANDCASTLES
I've seen this crossword clue in The Daily Telegraph
S
A
N
D
C
A
S
T
L
E
S
More clues leading to the result SANDCASTLES
Buddy explains!

The straight part
We know that "Keeps on the beach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keeps on the beach = sandcastles.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 SANDCASTLES mean?
SANDCASTLES noun- A sculpture made of sand and resembling a miniature castle; typically, but not always, made for fun by a child on a beach.