Alternative to reveille in Territorial Army (5)
I believe the answer is:
alarm
'alternative to reveille' is the definition.
(I've seen this before)
'in territorial army' is the wordplay.
'in' indicates a hidden word.
'ALARM' can be found hidden inside 'territorial army'.
(Other definitions for alarm that I've seen before include "Warning of danger" , "Distress signal" , "Fright" , "Frighten, scare" , "Scare with warning device" .)