Sovereign getting on within a month (7)
I believe the answer is:
monarch
'sovereign' is the definition.
(I know that sovereign can be written as monarch)
'on within a month' is the wordplay.
'within' indicates putting letters inside.
'a month' becomes 'March' (March is an example).
'on' inserted within 'march' is 'MONARCH'.
'getting' acts as a link.
(Other definitions for monarch that I've seen before include "Head of State" , "one over realm?" , "Sole hereditary ruler" , "Not a subject" , "Eg, king, queen, emperor" .)