The most common and most likely answer to this crossword clue is the 10 letter word RECIPROCAL
I've seen this crossword clue in The Guardian
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More clues leading to the result RECIPROCAL
Buddy explains!
The straight part
We know that "One of two is half/expressing mutual relationship" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of two is half/expressing mutual relationship = reciprocal.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 RECIPROCAL mean?
RECIPROCAL noun- The number obtained by dividing 1 by another given number; the result of exchanging the numerator and the denominator of a fraction.
- (grammar) A construction expressing mutual action.
- Of a feeling, action or such: mutual, uniformly felt or done by each party towards the other or others; two-way.
- Mutually interchangeable.
- (grammar) expressing mutual action, applied to pronouns and verbs; also in a broad sense: reflexive
- Used to denote different kinds of mutual relation; often with reference to the substitution of reciprocals for given quantities.
- Done, given, felt, or owed in return