The most common and most likely answer to this crossword clue is the 5 letter word ADDER
A
D
D
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More clues leading to the result ADDER
Buddy explains!

The straight part
We know that "A snake" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a snake = adder.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 ADDER mean?
ADDER noun- Any snake.
- A name loosely applied to various snakes more or less resembling the viper; a viper.
- Someone who or something which performs arithmetic addition; a machine for adding numbers.
- An electronic device that adds voltages, currents or frequencies.
- Something which adds or increases.