Small tree to become wet (4)
I believe the answer is:
soak
'wet' is the definition.
(soaking is a kind of wetting)
'small tree' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tree' becomes 'oak' (oak is a kind of tree).
's'+'oak'='SOAK'
'to become' acts as a link.
(Other definitions for soak that I've seen before include "Make thoroughly wet" , "Make extremely wet" , "Immerse in liquid" , "Habitual drinker (slang)" , "Steep in water" .)