The most common and most likely answer to this crossword clue is the 6 letter word THEORY
I've seen this crossword clue in Financial Times
T
H
E
O
R
Y
More clues leading to the result THEORY
- Those people pinch gold? That’s the hypothesis
- The man is taken in by right-wing supposition
- Eggs should be going up in your hypothesis
- Conjecture made by those people importing gold
- Conservative to accept governor’s hypothesis
- Conjecture the other ranks verify finally
- Guess what’s missing from Rossini’s Le Comte?
- Conservative welcoming the man’s idea
- Ambassador trapped in Conservative speculation
- Possible explanation from politician, full of gas
Buddy explains!

The straight part
We know that "speculation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case speculation = theory.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 THEORY mean?
THEORY noun- A description of an event or system that is considered to be accurate.
- Mental conception; reflection, consideration.
- A coherent statement or set of ideas that explains observed facts or phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc.
- The underlying principles or methods of a given technical skill, art etc., as opposed to its practice.
- A field of study attempting to exhaustively describe a particular class of constructs.
- A hypothesis or conjecture.
- A set of axioms together with all statements derivable from them; or, a set of statements which are deductively closed. Equivalently, a formal language plus a set of axioms (from which can then be derived theorems). The statements may be required to all be bound (i.e., to have no free variables).