The most common and most likely answer to this crossword clue is the 6 letter word THETAS
T
H
E
T
A
S
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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 THETAS mean?
THETAS noun- The eighth letter of the Modern Greek alphabet, ninth in Old Greek: Θ, θ.
- The measure of an angle.
- The sensitivity of the value of a derivative with respect to time; the "time decay".
- Designating a level of brain activity as measured by electroencephalography having a frequency of between four and seven cycles per second (associated with drowsiness in adults).
- The voiceless dental fricative represented by θ.