Back to update base with the old harmful look (4,3)
I believe the answer is:
evil eye
'harmful look' is the definition.
(I've seen this in another clue)
'back to update base with the old' is the wordplay.
'back to' suggests the final letters.
'base' becomes 'vile' (synonyms).
'with' says to put letters next to each other.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
The last letter of 'update' is 'e'.
'e'+'vile'+'ye'='EVIL-EYE'
(Other definitions for evil eye that I've seen before include "Malevolent look" , "Look with the power to inflict harm" , "Nasty look" , "Look supposed to have the power to harm" , "Bad-luck look" .)