11 across
Putting
two
lots
of
uranium
into
affected
corpuscles
is
dim!
(12)
Putting two lots of uranium into affected corpuscles is dim! (12)
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The definition is at the end of the clue.
How the clue works
Look at dim at the end — that's what you're after, meaning shadowy or twilit. Take CORPUSCLES, drop in two U's (uranium's symbol, and the clue gives you "two lots of uranium"), then anagram the whole lot ("affected") and you've got CREPUSCULOUS.