The most common and most likely answer to this crossword clue is the 13 letter word HOW-DO-YOU-DO
I've seen this crossword clue in The Guardian
H
O
W
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D
O
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Y
O
U
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D
O
More clues leading to the result HOW-DO-YOU-DO
Buddy explains!
The straight part
We know that "What to say on meeting/an awkward situation?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what to say on meeting/an awkward situation? = how-do-you-do.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.