growth (9)
(Other definitions for spurt that I've seen before include "Jet, gush" , "Gush out suddenly" , "Short spell of increased speed" , "A short burst of activity" , "Gush; sudden burst" .)
'growth' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for expansion that I've seen before include "increase in capacity" , "Enlargement" , "Process of increasing in size, volume etc" , "Increase in volume" , "fatter" .)