In Peru, let hero, ostensibly, reign (4,3,5)
I believe the answer is:
rule the roost
'reign' is the definition.
(thesaurus)
'in peru let hero ostensibly' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RULE THE ROOST' is hidden in the letters of 'peru let hero ostensibly'.
(Other definitions for rule the roost that I've seen before include "Be top cock" , "Dominate (as chief hen?)" , "Be in control" , "Be top hen" , "Rower" .)