Drench small tree (4)
I believe the answer is:
soak
'drench' is the definition.
(synonyms)
'small tree' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tree' becomes 'oak' (oak is a type of tree).
's'+'oak'='SOAK'
(Other definitions for soak that I've seen before include "Marinate" , "Habitual drinker (slang)" , "Steep in liquid" , "Make thoroughly wet" , "Steep (in fluid)" .)