The most common and most likely answer to this crossword clue is the 4 letter word ACID
I've seen this crossword clue in The Daily Telegraph
A
C
I
D
More clues leading to the result ACID
- Sharp Conservative stopping benefit
- Assistance required to cover top of charcoal burner
- Sharp girl about to go climbing
- One slice of damson on a cold tart
- Bitter is an intoxicating substance
- Drug produced by a police department
- “Cool”, but no place for “bitter”
- A police department rotter?
- There’s no place for mild bitter
- Police unit following a tart
Buddy explains!
The straight part
We know that "Keen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keen = acid.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 ACID mean?
ACID noun- A sour substance.
- Any of several classes of compound having the following properties:
- Lysergic acid diethylamide (LSD)
- Sour, sharp, or biting to the taste; tart; having the taste of vinegar.
- Sour-tempered.
- Of or pertaining to an acid; acidic.
- Denoting a musical genre that is a distortion (as if hallucinogenic) of an existing genre, as in acid house, acid jazz, acid rock.