The most common and most likely answer to this crossword clue is the 10 letter word RELATIVITY
I've seen this crossword clue in The Times
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More clues leading to the result RELATIVITY
Buddy explains!
The straight part
We know that "Subject of certain theories" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case subject of certain theories = relativity.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 RELATIVITY mean?
RELATIVITY noun- The state of being relative to something else.
- The principle that the laws of physics should be the same for all observers.
- Either of two theories (special relativity or general relativity) developed by German-American physicist Albert Einstein. Also called Einsteinian relativity.