Part of a pressure cooker, doubtless (6)
I believe the answer is:
proven
'part of a pressure cooker doubtless' is the definition.
I can't tell whether this definition defines the answer.
'part of a pressure cooker doubtless' is the wordplay.
'part of' indicates putting letters inside.
'a pressure' becomes 'p' (abbreviation).
'cooker' becomes 'oven' (synonyms).
'doubtless' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'p'+'oven'='poven'
'poven' placed around 'r' is 'PROVEN'.
Can you help me to learn more?
(Other definitions for proven that I've seen before include "Adjective meaning demonstrated beyond doubt" , "Established beyond doubt" , "Established with evidence" , "Conclusively demonstrated by evidence" .)