The head of the governors is in control as a rule (5)
I believe the answer is:
reign
'as a rule' is the definition.
(I've seen this before)
'the head of the governors is in control' is the wordplay.
'the head of' suggests taking the first letters.
'is in' is an insertion indicator.
'control' becomes 'rein' (reining is a kind of controlling).
The initial letter of 'governors' is 'g'.
'g' inserted into 'rein' is 'REIGN'.
(Other definitions for reign that I've seen before include "Sit on a throne" , "Sovereignty" , "Time on throne" , "period in power" , "Hegemony" .)