Quiet son becoming king - one should let off steam (6)
I believe the answer is:
kettle
'one should let off steam' is the definition.
(kettles let out steam)
'quiet son becoming king' is the wordplay.
I cannot quite see how this works, but
'king' could be 'k' (chess abbreviation) and 'k' is found in the answer.
The remaining letters 'ettle' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for kettle that I've seen before include "Eurydice's husband" , "Drum and metal pot" , "One boiled" , "Container for heating 10" , "Spouted vessel" .)