Natural hot spring (6)
I believe the answer is:
geyser
'natural hot spring' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for geyser that I've seen before include "Steaming spring" , "upsurge" , "Spring that discharges steam and hot water" , "It blows off steam" , "A spring sending up columns of hot water" .)