Leading lady rejected thermos left out, bizarrely, since the beginning (4,3,6)
I believe the answer is:
from the outset
'leading lady rejected thermos left out bizarrely' is the definition.
I know nothing about this answer so I can't judge whether this works.
'since the beginning' is the wordplay.
'since' becomes 'from'.
'beginning' becomes 'outset' (synonyms).
'from'+'the'+'outset'='FROM THE OUTSET'
Can you help me to learn more?
(Another definition for from the outset that I've seen is " Effective going right back to the start (of something)".)