Extracted from gum, a sticky substance (6)
I believe the answer is:
mastic
'substance' is the definition.
(mastic is a kind of substance)
'extracted from gum a sticky' is the wordplay.
'extracted from' says the answer is hidden in the clue.
'MASTIC' is hidden in the letters of 'gum a sticky'.
(Other definitions for mastic that I've seen before include "Substance applied to joints" , "Gum from trees" , "Waterproof sealant" , "liquor" , "Putty-like cement used as adhesive or filler" .)