Sovereign working in part of spring (7)
I believe the answer is:
monarch
'sovereign' is the definition.
('monarch' can be a synonym of 'sovereign')
'working in part of spring' is the wordplay.
'working in' becomes 'on' (I've seen this in other clues).
'part of' is an insertion indicator (part of the inside).
'spring' becomes 'march' (I've seen this before).
'on' inserted inside 'march' is 'MONARCH'.
(Other definitions for monarch that I've seen before include "Stag in Landseer picture" , "one over realm?" , "Eg, king, queen, emperor" , "Crowned head" , "Butterfly (emperor, say?)" .)