21 down
Material
for
path
of
serious
length
(6)
Material for path of serious length (6)
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The definition is at the beginning of the clue.
How the clue works
Material for path is your definition, pointing straight to a familiar surface you'd find on a driveway or garden walkway. The wordplay puts GRAVE (meaning serious) together with L (the standard abbreviation for length) to build the answer.