The most common and most likely answer to this crossword clue is the 15 letter word NONCONTRIBUTORY
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Buddy explains!
The straight part
We know that "add no value" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case add no value = noncontributory.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 NONCONTRIBUTORY mean?
NONCONTRIBUTORY adjective- Describing a pension plan in which the members do not provide their own direct contributions (that is, the employer makes all monetary contributions; the employees make no monetary contributions, whereas their service toward the company's ability to operate is their indirect contribution, for which the pension is remuneration).
- Not being a contributing factor (to a decision, analysis, or situation), such as not contributing to a medical diagnosis.