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' is the wordplay.
'part of a pressure' becomes 'pr' (I am not sure about this - if you are sure you should believe this answer much more).
'cooker' becomes 'oven' (I've seen this before).
'pr'+'oven'='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" .)