Stick face up (3)
I believe the answer is:
gum
'stick' is the definition.
(gum can mean to glue or stick)
'face up' is the wordplay.
'face' becomes 'mug' (informal term for face).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'mug' reversed gives 'GUM'.
(Other definitions for gum that I've seen before include "Sticky stuff - tree" , "Sticky substance and mouth tissue" , "To stick with adhesive" , "Eg, a eucalyptus" , "Adhesive for chewing" .)