The most common and most likely answer to this crossword clue is the 8 letter word THEORIST
I've seen this crossword clue in The Times
T
H
E
O
R
I
S
T
More clues leading to the result THEORIST
- One conjectures eggs will turn up in this nest ultimately
- Speculator ruined others with it
- Speculator in time before robbery having load of gold
- Article, golden, first for Einstein, say
- Thinker, hot, tires badly
- One reasoning believer accepts indication of alternative
- The detectorist leaves out notice for speculator
- Thinker’s article on wild riots
- One developing ideas in article on various riots
- Article on riots disturbed ideologue
Buddy explains!

The straight part
We know that "Speculator" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case speculator = theorist.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 THEORIST mean?
THEORIST noun- Someone who constructs theories, especially in the arts or sciences.