Don't give up (4)
'don't give up' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for keep that I've seen before include "Guard" , "Central tower of castle" , "Castle tower; cost of maintenance" , "Hold - part of castle" , "Retain or care for" .)
(Other definitions for ever that I've seen before include "Possibly" , "Continuously" , "Eternity" , "On any occasion" , "Perpetually" .)