Work out duty guarding monarch (8)
I believe the answer is:
exercise
'work out' is the definition.
('exercise' can be a synonym of 'work out')
'duty guarding monarch' is the wordplay.
'duty' becomes 'excise' (I've seen this before).
'guarding' means one lot of letters goes inside another.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'excise' going around 'er' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "Healthy activity" , "Task performed to develop skill" , "Do it to develop skill or fitness" , "Train, work out" , "problem" .)