The most common and most likely answer to this crossword clue is the 9 letter word ASYMPTOTE
I've seen this crossword clue in The Guardian
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More clues leading to the result ASYMPTOTE
Buddy explains!
The straight part
We know that "Line of approach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case line of approach = asymptote.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 ASYMPTOTE mean?
ASYMPTOTE noun- A straight line which a curve approaches arbitrarily closely, as they go to infinity. The limit of the curve, its tangent "at infinity".
- (by extension) Anything which comes near to but never meets something else.
- To approach, but never quite touch, a straight line, as something goes to infinity.