Plumber's problem (4)
I believe the answer is:
drip
'plumber's problem' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for drip that I've seen before include "Insubstantial person - trickle" , "Sissy" , "Leak slowly" , "Fall or let fall in drops" , "Liquid falling in small golbules, or a dull person" .)