The most common and most likely answer to this crossword clue is the 11 letter word RAZOR-SHARP
I've seen this crossword clue in The Guardian
R
A
Z
O
R
-
S
H
A
R
P
Buddy explains!
The straight part
We know that "Showing acute intelligence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing acute intelligence = razor-sharp.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 RAZOR-SHARP mean?
RAZOR-SHARP adjective- As sharp as a razor; extremely sharp.
- Very clever or quick-witted.