The most common and most likely answer to this crossword clue is the 5 letter word FLINT
I've seen this crossword clue in The Guardian
F
L
I
N
T
Buddy explains!

The straight part
We know that "11" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 11 = flint.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 FLINT mean?
FLINT noun- A hard, fine-grained quartz that fractures conchoidally and generates sparks when struck.
- A piece of flint, such as a gunflint, used to produce a spark by striking it with a firestriker.
- A small cylinder of some other material of the same function in a cigarette lighter, etc.
- A type of maize/corn with a hard outer hull.
- Anything figuratively hard.
- To furnish or decorate an object with flint.