Wet top of sycamore tree (4)
I believe the answer is:
soak
'wet' is the definition.
(soaking is a kind of wetting)
'top of sycamore tree' is the wordplay.
'top of' suggests taking the first letters.
'tree' becomes 'oak' (oak is a kind of tree).
The initial letter of 'sycamore' is 's'.
's'+'oak'='SOAK'
(Other definitions for soak that I've seen before include "Make extremely wet" , "Habitual drinker (slang)" , "Steep in liquid" , "Steep in water" , "Immerse in liquid" .)