London area vehicle's piercing alarm (5)
I believe the answer is:
scare
'alarm' is the definition.
(I know this)
'london area vehicle's piercing' is the wordplay.
'london area' becomes 'SE' (abbreviation for South East).
'vehicle' becomes 'car' (car is a kind of vehicle).
'piercing' indicates putting letters inside (some letters pierce a hole in others).
'se' enclosing 'car' is 'SCARE'.
(Other definitions for scare that I've seen before include "Panic" , "Greatly frighten" , "Health worry" , "Frighten or intimidate" , "Frighten, alarm" .)