Second unbelievable delay (5)
I believe the answer is:
stall
'delay' is the definition.
(stalling is a kind of delaying)
'second unbelievable' is the wordplay.
'second' becomes 's' (common abbreviation).
'unbelievable' becomes 'tall' (e.g. 'a tall story').
's'+'tall'='STALL'
(Other definitions for stall that I've seen before include "Seat - prevaricate" , "Part of the stable" , "Kiosk" , "Cowshed or stable" , "Box" .)