Vent one's feelings like a kettle (3,3,5)
I believe the answer is:
let off steam
'vent' is the definition.
(I've seen this in another clue)
'one's feelings like a kettle' is the wordplay.
I cannot really understand how this works, but
'one's' could be 'am' ('one is' can humorously mean 'I am') and 'am' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for let off steam that I've seen before include "Vent one's anger" , "no longer bottle it" , "Give vent to anger" , "Get rid of pent-up energy (colloq.)" , "Get rid of pent energy" .)