The most common and most likely answer to this crossword clue is the 8 letter word EIGHTIES
I've seen this crossword clue in The Times
E
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G
H
T
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S
Buddy explains!

The straight part
We know that "In which mercury’s rising — I see it’s fantastic outside" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in which mercury’s rising — i see it’s fantastic outside = eighties.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 EIGHTIES mean?
EIGHTIES noun- The decade of the 1880s, 1980s, etc.
- The decade of one's life from age 80 through age 89.
- (temperature, rates) The range between 80 and 89.
- From or evoking the 81st through 90th years of a century (chiefly the 1980s).