Back off (6)
'back off' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for relent that I've seen before include "Give in to influence or pressure" , "Be less strict" , "Relax severity or opposition" , "Become less severe, soften" , "Decide to be kind after all?" .)
(Other definitions for retreat that I've seen before include "Monastery - bugle call" , "Leave - refuge" , "hermitage, perhaps" , "Tree rat (anag)" , "concession" .)