Press down, as pipe tobacco (4)
I believe the answer is:
tamp
'press down as pipe tobacco' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tamp that I've seen before include "press down firmly" , "Firmly ram down" , "Pack (eg earth) down firmly" , "Pack or ram down tightly" , "Press down tightly" .)