Alarming disturbance on the border! (8)
I believe the answer is:
marginal
'on the border' is the definition.
I can't tell whether this defines the answer.
'alarming disturbance' is the wordplay.
'disturbance' indicates anagramming the letters.
'alarming' anagrammed gives 'MARGINAL'.
Can you help me to learn more?
(Other definitions for marginal that I've seen before include "Very slight in amount, at the edge" , "unsafe seat" , "Alarming (anag) - too small to matter" , "Peripheral in alarming way" , "Borderline" .)