Ultimate authority on electronic signal boxes (9)
I believe the answer is:
'ultimate authority' is the definition.
'on electronic signal boxes' is the wordplay.
I cannot really understand how this works, but
'on' could be 'over' (I've seen this in other clues) and 'over' is found in the answer.
'electronic' could be 'e' (as in e-mail or e-books) and 'e' is present in the answer.
'signal' could be 'sign' (synonyms) and 'sign' is found in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for sovereign that I've seen before include "Old money" , "Supreme ruler" , "Her Maj?" , "more than a prince" , "Ruler - gold coin" .)