The most common and most likely answer to this crossword clue is the 11 letter word STREAMLINED
I've seen this crossword clue in The Daily Telegraph
S
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More clues leading to the result STREAMLINED
Buddy explains!

The straight part
We know that "efficient" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case efficient = streamlined.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 STREAMLINED mean?
STREAMLINED verb- To design and construct the contours of a vehicle etc. so as to offer the least resistance to its flow through a fluid.
- (by extension) To simplify or organize a process in order to increase its efficiency.
- To modernise.
- Designed to offer little resistance to the flow of fluid, especially by having sleek, graceful lines.
- Having been made more simple and straight forward.