Penetrate slowly (4)
I believe the answer is:
seep
'penetrate slowly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seep that I've seen before include "Trickle through slowly" , "Leak gradually" , "Leak slowly, ooze" , "Flow or leak slowly" , "Ooze, percolate" .)