13 down
Perhaps
silk
handkerchief
is
irrelevant
(3,2,5)
Perhaps silk handkerchief is irrelevant (3,2,5)
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The definition is at the end of the clue.
How the clue works
The definition is irrelevant, which matches NOT AT ISSUE. The wordplay reads 'Perhaps silk handkerchief' as something that is NOT A TISSUE (since a silk handkerchief is not a paper tissue), and the same letters respaced give NOT AT ISSUE.