The most common and most likely answer to this crossword clue is the 8 letter word FRANKING
I've seen this crossword clue in The Daily Telegraph
F
R
A
N
K
I
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G
Buddy explains!
The straight part
We know that "Source of legitimacy in post" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of legitimacy in post = franking.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 FRANKING mean?
FRANKING verb- To place a frank on an envelope.
- To exempt from charge for postage, as a letter, package, or packet, etc.
- To send by public conveyance free of expense.
- To shut up in a frank or sty; to pen up; hence, to cram; to fatten.
- A device or marking such as postage stamp, printed or stamped impressions, codings, labels, manuscript writings, or any other authorized form of markings affixed or applied to mails to qualify them to be postally serviced.
- A corporate tax system in which some or all of the tax paid by a company may be attributed, or imputed, to the shareholders by way of a tax credit to reduce the income tax payable on a distribution.