Look, keep up! (4)
I believe the answer is:
peek
'look' is the definition.
(peek is a kind of look)
'keep up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'keep' reversed gives 'PEEK'.
(Other definitions for peek that I've seen before include "Secret look" , "Quick, sly look" , "spy" , "Quick or furtive look" , "Furtive glance" .)