Hot spring (6)
I believe the answer is:
geyser
'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 "Spring that spouts hot water" , "Gas-fired water heater" , "Spring spouting steam and hot water" , "It gushes steam and hot water" , "Spring that discharges steam and hot water" .)