Begin with heather? Alarming! (9)
I believe the answer is:
startling
'alarming' is the definition.
(I've seen this in another clue)
'begin with heather?' is the wordplay.
'begin' becomes 'start' (synonyms).
'with' is a charade indicator (letters next to each other).
'heather?' becomes 'ling' ('ling' can be a synonym of 'heather').
'start'+'ling'='STARTLING'
(Other definitions for startling that I've seen before include "Rattlings of alarming kind" , "Remarkable" , "Suddenly frightening" , "Surprising, shocking" , "Producing sudden alarm" .)