Be about to happen (6)
I believe the answer is:
impend
'be about to happen' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for impend that I've seen before include "Be about to take place" , "Be looming" , "(Of a bad thing) loom" , "appearing to be imminent" , "Threaten - be about to happen" .)