Never start up with a ring of fire here, perhaps. (4)
I believe the answer is:
oven
'never start up with a ring of fire here perhaps' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'of fire here perhaps' is the wordplay.
'of' becomes 'o''.
'fire here perhaps' becomes 'ven' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o'+'ven'='OVEN'
Can you help me to learn more?
(Other definitions for oven that I've seen before include "Section of a cooker" , "Stove" , "Part of a cooker" , "Place for burning" , "It may get very hot" .)