18 across
Starts
off
by
installing
one
sensible
kind
of
clock
(10)
Starts off by installing one sensible kind of clock (10)
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The definition is at the end of the clue.
How the clue works
Look at kind of clock at the end — that's your definition, the biological sort your body runs on. Take the first letters ("Starts off") of By Installing One and you get BIO, then sensible gives you LOGICAL, and the two parts slot together nicely.