Head of Government in control for period in office (5)
I believe the answer is:
reign
'period in office' is the definition.
(eg the reign or tenure of a prime minister)
'head of government in control' is the wordplay.
'head of' indicates taking the first letters.
'in' indicates putting letters inside.
'control' becomes 'rein' (reining is a kind of controlling).
The initial letter of 'government' is 'g'.
'g' put within 'rein' is 'REIGN'.
'for' is the link.
(Other definitions for reign that I've seen before include "Govern" , "Prevail" , "Hold sway, rule" , "Monarch's time in the job" , "Monarch's period" .)