What a great time Deb and Rob and Seb all have! (5)
I believe the answer is:
blast
'what a great time' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for blast that I've seen before include "Report" , "slate" , "Great time: explosion" , "Shoot" , "Severely criticise" .)