Look, hold up! (4)
I believe the answer is:
peek
'look' is the definition.
(peek can mean to look at quickly)
'hold up' is the wordplay.
'hold' becomes 'keep' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'keep' written backwards gives 'PEEK'.
(Other definitions for peek that I've seen before include "Glimpse" , "Quick, sly look" , "Secret look" , "Furtive glance" , "Sly look, peep" .)