It could be alarming at the edge (8)
I believe the answer is:
marginal
'at the edge' is the definition.
'it could be alarming' is the wordplay.
'it could be' is an anagram indicator (the same letters could be an anagram).
'alarming' anagrammed gives 'MARGINAL'.
(Other definitions for marginal that I've seen before include "Alarming (anag) - too small to matter" , "unsafe seat" , "having small majority at risk" , "On the edges of" , "Very slight in amount, at the edge" .)