An exceptionally warm spring (6)

I believe the answer is:

geyser

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'spring' is the definition.
(a geyser is a naturally forming hot spring)

I can't explain the remainder of the clue.

(Other definitions for geyser that I've seen before include "Intermittent hot spring" , "upsurge" , "Spring that discharges steam and hot water" , "It blows off steam" , "Spring that spouts hot water" .)

I've seen this clue in The Globe and Mail.

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