The most common and most likely answer to this crossword clue is the 6 letter word MOTTLE
I've seen this crossword clue in Financial Times
M
O
T
T
L
E
Buddy explains!
The straight part
We know that "Spot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spot = mottle.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 MOTTLE mean?
MOTTLE noun- A distinguishing blotch of color.
- A mottled coloration or pattern.
- To mark with blotches of different color, or shades of color, as if stained; to spot; to maculate.