Waters small trees (5)
I believe the answer is:
soaks
'waters' is the definition.
(soak can mean to wet or water)
'small trees' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'trees' becomes 'oaks' (oak is a kind of tree).
's'+'oaks'='SOAKS'
(Other definitions for soaks that I've seen before include "Drenches" , "big downers" , "Saturates" .)