Power of Government head in control (5)
I believe the answer is:
reign
'power' is the definition.
(thesaurus)
'government head in control' is the wordplay.
'head' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'control' becomes 'rein' (rein is a kind of control**).
The initial letter of 'government' is 'g'.
'g' going into 'rein' is 'REIGN'.
'of' is the link.
(Other definitions for reign that I've seen before include "Have sovereign power" , "Wield the sceptre" , "Spell in office" , "Period on the throne" , "Rule of monarch" .)