Period showing king ultimately in control? (5)
I believe the answer is:
reign
'period' is the definition.
(reign is a kind of period)
'king ultimately in control?' is the wordplay.
'ultimately' says to take the final letters.
'in' indicates putting letters inside.
'control?' becomes 'rein' (rein is a kind of control**).
The final letter of 'king' is 'g'.
'g' inserted into 'rein' is 'REIGN'.
'showing' is the link.
(Other definitions for reign that I've seen before include "Dominion" , "Period of a monarchy" , "Rule over in Niger" , "Sit on a throne" , "Monarch's rule" .)