Literature problem raised in sort of test paper (6)
I believe the answer is:
litmus
'sort of test paper' is the definition.
'literature problem raised' is the wordplay.
'literature' becomes 'lit'.
'problem' becomes 'sum' (arithmetic problem).
'raised' says the letters should be written backwards.
'sum' in reverse letter order is 'mus'.
'lit'+'mus'='LITMUS'
'in' acts as a link.
(Other definitions for litmus that I've seen before include "It turns red in acid and blue in alkali" , "Chemistry lab indicator" , "Substance used to tell acid from alkali" , "Substance used in test for acidity" , "paper for chemistry test" .)