Delay unbelievable after standing start (5)
I believe the answer is:
stall
'delay' is the definition.
(stalling is a kind of delaying)
'unbelievable after standing start' is the wordplay.
'unbelievable' becomes 'tall' (e.g. 'a tall story').
'after' means one lot of letters go next to another.
'start' suggests taking the first letters.
The first letter of 'standing' is 's'.
'tall' after 's' is 'STALL'.
(Other definitions for stall that I've seen before include "outlet at a fair" , "Kiosk" , "Equivocate" , "Theatre seat" , "Hedge" .)