The most common and most likely answer to this crossword clue is the 11 letter word PROTRACTORS
I've seen this crossword clue in The Daily Telegraph
P
R
O
T
R
A
C
T
O
R
S
Buddy explains!
The straight part
We know that "Mathematical devices" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mathematical devices = protractors.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 PROTRACTORS mean?
PROTRACTORS noun- One who, or that which, protracts, or causes protraction.
- A circular or semicircular tool for drawing or measuring angles.
- An instrument formerly used in extracting foreign or offensive matter from a wound.
- A muscle that extends an organ or part; opposed to retractor.
- An adjustable pattern used by tailors.