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

The straight part
We know that "mutually beneficial relationship" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mutually beneficial relationship = symbiosis.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 SYMBIOSIS mean?
SYMBIOSIS noun- A relationship of mutual benefit, especially among different species.
- A close, prolonged association between two or more organisms of different species, regardless of benefit to the members.
- (possibly obsolete) The state of people living together in a community.