Alarm about boarding offence (5)
I believe the answer is:
siren
'alarm' is the definition.
(I know that siren is a type of alarm)
'about boarding offence' is the wordplay.
'about' becomes 're' (regarding).
'boarding' is an insertion indicator.
'offence' becomes 'sin' (a sin is an offence or transgression).
're' placed inside 'sin' is 'SIREN'.
(Other definitions for siren that I've seen before include "Temptress - makes a loud noise!" , "Alluring female" , "Warning device, alluring woman" , "Device making a loud wailing sound" , "Device producing a loud wailing sound as a signal" .)