Cryptic crossword clue
Tin and lead fused in scientific unit 7

The most common and most likely answer to this crossword clue is the 7 letter word CANDELA

I've seen this crossword clue in The Guardian
C
A
N
D
E
L
A

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The straight part
We know that "scientific unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case scientific unit = candela.
The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.
This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.

What does CANDELA mean?

CANDELA noun
  1. In the International System of Units, the base unit of luminous intensity; the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. Symbol: cd

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