Sudden rush, after wasting time, to be prompt (4)
I believe the answer is:
spur
'be prompt' is the definition.
(I've seen this before)
'sudden rush after wasting time' is the wordplay.
'sudden rush' becomes 'spurt' (a spurt is a sudden burst or jet).
'after wasting' suggests deleting specific letters.
'time' becomes 't' (abbreviation).
'spurt' with 't' removed is 'SPUR'.
'to' acts as a link.
(Other definitions for spur that I've seen before include "Sharply pointed projection" , "outcrop" , "railway siding" , "'Urge, goad (4)'" , "Goad; stimulus" .)