The most common and most likely answer to this crossword clue is the 8 letter word EFFICACY
I've seen this crossword clue in Independent
E
F
F
I
C
A
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Y
More clues leading to the result EFFICACY
Buddy explains!
The straight part
We know that "ability to produce desired results" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ability to produce desired results = efficacy.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 EFFICACY mean?
EFFICACY noun- Ability to produce a desired effect under ideal testing conditions.
- Degree of ability to produce a desired effect.