Flow as some water underground (3)
I believe the answer is:
run
'flow' is the definition.
(I know that run is a type of flow)
'some water underground' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'RUN' can be found hidden inside 'water underground'.
'as' is the link.
(Other definitions for run that I've seen before include "Flow quickly" , "Administer" , "Go swiftly - ladder" , "Operate; flow" , "Hasten" .)