Definite article or beginning of yet unproven supposition (6)
I believe the answer is:
theory
'unproven supposition' is the definition.
'theory' can be an answer for 'supposition' (I've seen this before). I'm unsure of the 'unproven' bit.
'definite article or beginning of yet' is the wordplay.
'definite article' becomes 'the' (I've seen this before).
'beginning of' says to take the initial letters.
The first letter of 'yet' is 'y'.
'the'+'or'+'y'='THEORY'
Can you help me to learn more?
(Other definitions for theory that I've seen before include "Hypothesis or concept" , "Set of ideas formulated to explain something" , "Supposition, set of ideas" , "Speculative view" , "Hypothetical reasoning" .)