Imminent (7)
(Other definitions for looming that I've seen before include "Hovering as something dark or threatening" , "Imminent in a menacing way" , "Coming into view" , "Appearing indistinctly" , "Appearing threateningly" .)
'imminent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pending that I've seen before include "Awaiting settlement" , "Awaiting action" , "near" , "Outstanding" , "undecided" .)
(Other definitions for horizon that I've seen before include "Line where sky meets sea" , "Where the sky appears to meet the earth" , "As far as one can see" , "As far as the eye can see" , "The line at which the sky and the earth appear to meet" .)