Sudden increase certain to restrict government initially (5)
I believe the answer is:
surge
'sudden increase' is the definition.
(a surge is a sudden increase in something)
'certain to restrict government initially' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to restrict' indicates putting letters inside (inserted letters are restricted or trapped).
'initially' says to take the initial letters.
The first letter of 'government' is 'g'.
'sure' enclosing 'g' is 'SURGE'.
(Other definitions for surge that I've seen before include "Powerful rush (of emotion)" , "Sudden rush" , "A great wave" , "Rise suddenly" , "Sudden upward movement" .)