The most common and most likely answer to this crossword clue is the 7 letter word LONGEST
I've seen this crossword clue in The Sunday Telegraph
L
O
N
G
E
S
T
More clues leading to the result LONGEST
- Most extensive record about knight is French
- Like September of all months? Yes and no
- Biggest ten logs chopped up
- Slowest legs not abnormal
- In case being around group's not good, it will be most protracted
- Look, new hotel customer, not superior, is most tedious
- Arrange ten logs having maximum length
- Tune at first followed loud grating song, taking even more time than any other
Buddy explains!

The straight part
We know that "Like September of all months? Yes and no" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like september of all months? yes and no = longest.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 LONGEST mean?
LONGEST verb- To take a long position in.
- Having much distance from one terminating point on an object or an area to another terminating point (usually applies to horizontal dimensions; see Usage Notes below).
- Having great duration.
- Seemingly lasting a lot of time, because it is boring or tedious or tiring.
- Not short; tall.
- Possessing or owning stocks, bonds, commodities or other financial instruments with the aim of benefiting of the expected rise in their value.
- Of a fielding position, close to the boundary (or closer to the boundary than the equivalent short position).
- (of a ball or a shot) Landing beyond the baseline, and therefore deemed to be out.
- Occurring or coming after an extended interval; distant in time; far away.
- Over a great distance in space.
- For a particular duration.
- For a long duration.
- To await, aspire, desire greatly (something to occur or to be true)
- To be appropriate to, to pertain or belong to.
- To belong.