Cryptic crossword clue
Various units entering flat 13

The most common and most likely answer to this crossword clue is the 13 letter word UNINTERESTING

I've seen this crossword clue in The Daily Telegraph
U
N
I
N
T
E
R
E
S
T
I
N
G

More clues leading to the result UNINTERESTING

Buddy explains!

The straight part
We know that "flat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flat = uninteresting.
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 UNINTERESTING mean?

UNINTERESTING adjective
  1. Arousing little or no interest; boring or uneventful.

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