Small tree that's wet through (4)
I believe the answer is:
soak
'wet through' is the definition.
'soak' can be an answer for 'wet' (soaking is a kind of wetting). I'm not certain of the 'through' bit.
'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'
'that's' is the link.
Can you help me to learn more?
(Other definitions for soak that I've seen before include "Steep in water" , "Immerse in liquid" , "Douse, drench" , "Steep (in fluid)" , "Make thoroughly wet" .)