Y-reg Seat half rebuilt in spring (6)
I believe the answer is:
geyser
'spring' is the definition.
(geyser is a kind of spring)
'y-reg seat half rebuilt' is the wordplay.
'half' means to take half the letters.
'rebuilt' indicates an anagram.
'seat' with half the letters taken is 'se'.
'y'+'reg'+'se'='yregse'
'yregse' anagrammed gives 'GEYSER'.
'in' acts as a link.
(Other definitions for geyser that I've seen before include "A spring sending up columns of hot water" , "Natural spout of hot water" , "upsurge" , "Intermittent hot spring" , "A jet of hot water from natural spring" .)