The most common and most likely answer to this crossword clue is the 6 letter word PLANET
P
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More clues leading to the result PLANET
- Mercury, say, element at outset brought into factory
- Factory stocking English revolver?
- Earth, say, in tree found by tribe's leader
- Scheme with alien world
- Aim SF film — to be set here?
- Earth, say, where flower perhaps stores energy
- Way to load part-emptied revolver?
- Earth perhaps flat? Thinking initially
- Factory containing last piece of satellite for Mars, perhaps
- Alien after map for Mars, say?
Buddy explains!

The straight part
We know that "Mercury, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mercury, say = planet.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 PLANET mean?
PLANET noun- Each of the seven major bodies which move relative to the fixed stars in the night sky—the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn.
- A body which orbits the Sun directly and is massive enough to be in hydrostatic equilibrium (effectively meaning a spheroid) and to dominate its orbit; specifically, the eight major bodies of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (Pluto was considered a planet until 2006 and has now been reclassified as a dwarf planet.)
- A large body which directly orbits any star (or star cluster) but which has not attained nuclear fusion.
- In phrases such as the planet, this planet, sometimes refers to the Earth.