Reign is finished within borders of Gabon (6)
I believe the answer is:
govern
'reign' is the definition.
(thesaurus)
'finished within borders of gabon' is the wordplay.
'finished' becomes 'over' ('over' can be similar in meaning to 'finished').
'within' indicates putting letters inside.
'borders of' suggests removing the centre.
'gabon' with its middle taken out is 'gn'.
'over' going within 'gn' is 'GOVERN'.
'is' is the link.
(Other definitions for govern that I've seen before include "Control, determine" , "Rule or administer" , "Exercise authority over a people" , "Manage - control" , "Be in power" .)