Cryptic crossword clue
Son entertained by silly ‘alien’ pun — ‘one that sticks out on Earth’ 9

The most common and most likely answer to this crossword clue is the 9 letter word PENINSULA

I've seen this crossword clue in The Daily Telegraph
P
E
N
I
N
S
U
L
A


Buddy explains!

The straight part
We know that "‘one that sticks out on Earth’" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ‘one that sticks out on earth’ = peninsula.
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 PENINSULA mean?

PENINSULA noun
  1. A piece of land projecting into water from a larger land mass.

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