Begin extremely late causing alarm (7)
I believe the answer is:
startle
'alarm' is the definition.
(I've seen this before)
'begin extremely late' is the wordplay.
'begin' becomes 'start' (synonyms).
'extremely' means to remove the middle letters.
'late' with its centre taken out is 'le'.
'start'+'le'='STARTLE'
'causing' acts as a link.
(Other definitions for startle that I've seen before include "Cause shock/alarm" , "Astonish" , "Make jump" , "Surprise with fright" , "Rock" .)