11 across
Tramp
beside
Cambridge
river
taking
in
one
feature
of
arboreal
growth
(7)
Tramp beside Cambridge river taking in one feature of arboreal growth (7)
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The definition is at the end of the clue.
How the clue works
Right, so feature of arboreal growth is your definition there — that's pointing to the cambium, the layer under tree bark. For the wordplay, CAM is the Cambridge river, and BUM is your "tramp" - stick I (that's "one") inside BUM to get BIUM, then park CAM beside it and you've got CAMBIUM.