Sudden increase (5)
'sudden increase' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for upsurge that I've seen before include "Sudden rapid increase" , "Sudden or abrupt strong increase" , "Sudden forceful flow from below" , "Effect of groundswell" , "Rapid rise" .)
'sudden increase' is the definition.
(a surge is a sudden increase in something)
This is all the clue.
(Other definitions for surge that I've seen before include "Powerful rush (of emotion)" , "Sudden flow" , "Sudden onward motion" , "Sudden powerful increase" , "significant increase" .)