The most common and most likely answer to this crossword clue is the 4 letter word SURD
S
U
R
D
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The straight part
We know that "Irrational" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case irrational = surd.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 SURD mean?
SURD noun- An irrational number, especially one expressed using the √ symbol.
- A voiceless consonant.
- Lacking the sense of hearing; deaf.
- Unheard
- Involving surds, or irrational numbers; not capable of being expressed in rational numbers.
- Unvoiced; voiceless