The most common and most likely answer to this crossword clue is the 9 letter word CONJUGATE
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More clues leading to the result CONJUGATE
Buddy explains!

The straight part
We know that "Coupled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coupled = conjugate.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 CONJUGATE mean?
CONJUGATE noun- Any entity formed by joining two or more smaller entities together.
- (of a complex number) A complex conjugate.
- More generally, any of a set of irrational or complex numbers that are zeros of the same polynomial with integral coefficients.
- (of an element of an extension field) Given a field extension L / K and an element α ∈ L, any other element β ∈ L that is another root of the minimal polynomial of α over K.
- An explementary angle.
- (grammar) A word agreeing in derivation with another word, and therefore generally resembling it in meaning.
- A weak and a strong antigen covalently linked together
- (grammar) To inflect (a verb) for each person, in order, for one or more tenses.
- To multiply on the left by one element and on the right by its inverse.
- To join together, unite; to juxtapose.
- (of bacteria and algae) To temporarily fuse, exchanging or transferring DNA.
- United in pairs; yoked together; coupled.
- In single pairs; coupled.
- Containing two or more radicals supposed to act the part of a single one.
- (grammar) Agreeing in derivation and radical signification; said of words.
- Presenting themselves simultaneously and having reciprocal properties; said of quantities, points, lines, axes, curves, etc.