Stick something in one’s mouth (3)
I believe the answer is:
gum
'mouth' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for gum that I've seen before include "A chopper base" , "Prepartion for sticking or chewing" , "Chewed sweet" , "Sticky stuff - tree" , "Adhesive - eucalyptus" .)