16 across
I
don’t
believe
helium
is
found
in
a
nucleus
of
matter
(7)
I don’t believe helium is found in a nucleus of matter (7)
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The definition is at the beginning of the clue.
How the clue works
I don't believe defines ATHEIST, someone who lacks belief. The wordplay takes A plus TT (the nucleus, or centre, of "matter") and inserts HE (helium) plus IS into it, giving A-T-(HEIS)-T = ATHEIST.