Yes, pretty standard clause. If you've said you have an approved alarm, but don't set it, then they can refuse to pay or apply a much higher excess. The alarm must also be maintained properly - insurance fine print usually means an annual service.
Aviva for example double the excess if alarm wasn't set:
d the alarm is set and working when there is
no person at home.
In the event that the alarm system is not in
operation during a theft or attempted theft,
the excess applying to the policy is doubled for that claim."