The most common and most likely answer to this crossword clue is the 5 plus 4 letter word SUGAR CANE
I've seen this crossword clue in The Times
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More clues leading to the result SUGAR CANE
- Theatre cat rejected mysterious crop
- Oddly stung over secret source of sweetener
- Bizarrely, US carnage results in something sweet
- Get a gun and scare bats in plantation crop
- Rising American government’s secret plant
- Grand American brought back mysterious crop from tropics
- Angus turned up with mysterious source of sweetness
- Angus brought back esoteric sweet stuff
- Grass suing when not in on secret
- Try to sell secret source of sweetener
Buddy explains!

The straight part
We know that "source of sweetener" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of sweetener = sugar cane.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 SUGAR CANE mean?
SUGAR CANE noun- A tropical grass of the genus Saccharum (especially the species Saccharum officinarum) having stout, fibrous, jointed stalks, the sap of which is a source of sugar.
- An edible candy in the shape of a cane.