Cryptic crossword clue
One that can’t stand returning for amino acid? 4

The most common and most likely answer to this crossword clue is the 4 letter word DOPA

D
O
P
A

Buddy explains!

The straight part
We know that "amino acid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case amino acid = dopa.
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 DOPA mean?

DOPA noun
  1. The amino acid dihydroxyphenylalanine that is generated in the liver from tyrosine and then converted into dopamine in the brain.

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