Cryptic crossword clue
This will remove tea stains — but one needs to verify 12

The most common and most likely answer to this crossword clue is the 12 letter word SUBSTANTIATE

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

Buddy explains!

The straight part
We know that "one needs to verify" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one needs to verify = substantiate.
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 SUBSTANTIATE mean?

SUBSTANTIATE verb
  1. To verify something by supplying evidence; to authenticate or corroborate
  2. To give material form or substance to something; to embody; to record in documents

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