Prevention keeps up at any time (4)
I believe the answer is:
ever
'at any time' is the definition.
(synonyms)
'prevention keeps up' is the wordplay.
'keeps' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'reve' is hidden within 'prevention'.
'reve' backwards is 'EVER'.
(Other definitions for ever that I've seen before include "Eternity" , "Veer away endlessly" , "Perpetually" , "In perpetuity" , "Continuously" .)