Outspoken man in spring letting off steam (6)
I believe the answer is:
geyser
'steam' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for geyser that I've seen before include "It blows off steam" , "one named Old Faithful" , "spouter!" , "Steaming spring" , "upsurge" .)