Pack (down) (4)
I believe the answer is:
tamp
'pack down' 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 "Pack (eg earth) down firmly" , "Pack down eg tobacco in a pipe" , "Press, pack (down)" , "Push down tobacco" , "press down firmly" .)