So long, some essay on a rainforest (8)
I believe the answer is:
sayonara
'so long' is the definition.
('sayonara' can be a synonym of 'so long')
'some essay on a rainforest' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'SAYONARA' is hidden in the letters of 'essay on a rainforest'.
(Other definitions for sayonara that I've seen before include "Japanese goodbye" , "So long" , "Goodbye (Jap.)" , "Japanese farewell" , "Japanese valediction" .)