The most common and most likely answer to this crossword clue is the 6 letter word NOZZLE
I've seen this crossword clue in The Guardian
N
O
Z
Z
L
E
Buddy explains!

The straight part
We know that "dispenser of 21" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dispenser of 21 = nozzle.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 NOZZLE mean?
NOZZLE noun- A short tube, usually tapering, forming the vent of a hose or pipe.
- A short outlet or inlet pipe projecting from the end or side of a hollow vessel, as a steam-engine cylinder or a steam boiler.
- The nose of an animal; muzzle.
- The part of an earbud that accomodates eartips.