The most common and most likely answer to this crossword clue is the 5 letter word MARDY
M
A
R
D
Y
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The straight part
We know that "irritable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case irritable = mardy.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 MARDY mean?
MARDY noun- A sulky, whiny mood; a fit of petulance.
- Sulky or whining.
- Non-co-operative, bad-tempered or terse in communication.