The most common and most likely answer to this crossword clue is the 5 plus 2 letter word STRAP ON
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Buddy explains!
The straight part
We know that "Fasten" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fasten = strap on.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 STRAP ON mean?
STRAP ON noun- An artificial phallus attached with a strap, usually a dildo.