Prince about to get on alert system? (5)
I believe the answer is:
pager
'alert system?' is the definition.
(device for sending and receiving alerts)
'prince about to get on' is the wordplay.
'prince' becomes 'pr'.
'about' means one lot of letters goes inside another.
'to get on' becomes 'age' (synonyms).
'pr' placed around 'age' is 'PAGER'.
(Other definitions for pager that I've seen before include "One calling" , "communicator?" , "Old electronic bleeping device" , "message on this?" , "Contact device" .)