Test paper (5)
I believe this clue is a double definition.
'test' is the first definition.
('essay' can be a synonym of 'test')
'paper' is the second definition.
(I've seen this before)
(Other definitions for essay that I've seen before include "Try; short written work" , "School composition" , "Analytic or interpretive literary composition (5_" , "Scholarly work" , "Endeavour" .)
'test paper' is the definition.
(litmus is a kind of test paper)
This is all the clue.
(Other definitions for litmus that I've seen before include "It turns red with acids and blue with alkalies" , "A dye used as a rough indicator of acid" , "It turns red in acid and blue in alkali" , "Acid-test paper" , "Acid-alkali testing substance" .)