Worry king’s trapped by duty (8)
I believe the answer is:
exercise
'worry' is the definition.
(I've seen this before)
'king's trapped by duty' is the wordplay.
'king' becomes 'ER' (abbreviation for Edward Rex, various English kings).
'trapped by' means one lot of letters goes inside another.
'duty' becomes 'excise' (both can mean a tax).
'er' inserted within 'excise' is 'EXERCISE'.
(Other definitions for exercise that I've seen before include "Do it to get fit" , "Train, work out" , "Practice session" , "Keep-fit" , "Physical training or exertion" .)