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Forward
–
as
one’s
behind?
(6)
Forward – as one’s behind? (6)
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The whole clue is the definition (& lit or double definition).
How the clue works
Forward means impudent or cheeky, while as one's behind? jokingly points to the buttocks, which are made up of cheeks. The two definitions together both yield CHEEKY, the second being a playful pun on bottom cheeks.