Leaked gradually through a small opening (6)
I believe the answer is:
seeped
'leaked gradually through a small opening' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for seeped that I've seen before include "Oozed or percolated" , "Exuded" , "Sweated" , "Leaked through small openings" , "Passed gradually or leaked through" .)