Terribly moist leaves out! (5)
I believe the answer is:
omits
'leaves out' is the definition.
('omit' can be a synonym of 'leave out')
'terribly moist' is the wordplay.
'terribly' indicates an anagram.
'moist' is an anagram of 'OMITS'.
(Other definitions for omits that I've seen before include "overlooks" , "misses out" , "Leaves out or neglects" , "skips" , "Forgets" .)