Ponder, with "on" (4)
I believe the answer is:
chew
'ponder with on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for chew that I've seen before include "champ" , "Work or grind between teeth" , "work with canines" , "Bite and grind food in the mouth" , "Munch" .)