Mug up something sticky (3)
I believe the answer is:
gum
'something sticky' is the definition.
'mug up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'mug' reversed gives 'GUM'.
(Other definitions for gum that I've seen before include "A chopper base" , "Part of mouth; a tree" , "Adhesive for chewing" , "Prepartion for sticking or chewing" , "Gingeva is its technical name" .)