The most common and most likely answer to this crossword clue is the 5 letter word CHORD
C
H
O
R
D
Buddy explains!
The straight part
We know that "straight line inside a 9 Ac." is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case straight line inside a 9 ac. = chord.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 CHORD mean?
CHORD noun- A harmonic set of three or more notes that is heard as if sounding simultaneously.
- A straight line between two points of a curve.
- A horizontal member of a truss.
- The distance between the leading and trailing edge of a wing, measured in the direction of the normal airflow.
- An imaginary line from the luff of a sail to its leech.
- A keyboard shortcut that involves two or more distinct keypresses, such as Ctrl+M followed by P.
- The string of a musical instrument.
- A cord.
- An edge that is not part of a cycle but connects two vertices of the cycle.
- To write chords for.
- To accord; to harmonize together.
- To provide with musical chords or strings; to string; to tune.