Mighty sovereign in traction (8)
I believe the answer is:
towering
'mighty' is the definition.
(thesaurus)
'sovereign in traction' is the wordplay.
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'in' is an insertion indicator.
'traction' becomes 'towing' (traction means dragging or towing along).
'er' going inside 'towing' is 'TOWERING'.
(Other definitions for towering that I've seen before include "Very tall" , "I get worn (anag)" , "Very intense (rage)" , "Grew into (anag.)" , "Magnificent" .)