17 across
Tree
having
less
on
top
is
topped
(5)
Tree having less on top is topped (5)
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The definition is at the beginning of the clue.
How the clue works
The definition is Tree — specifically an alder. Having less on top gives you BALDER (as in, more bald), and then is topped tells you to lop off the first letter, leaving you with ALDER.