Dawn Basra raid viable, every now and then (7)
I believe the answer is:
arrival
'dawn' is the definition.
(both can mean the beginning or advent of something)
'basra raid viable every now and then' is the wordplay.
'every now and then' means one should take alternating letters (letters selected at regular intervals).
'basra'+'raid'+'viable'='basraraidviable'
The alternate letters of 'basraraidviable' are 'ARRIVAL'.
(Other definitions for arrival that I've seen before include "Achievement of destination" , "Entry" , "Attainment of destination" , "The A in ETA" , "Advent" .)