It produces reaction — litmus changes before its conclusion (8)
I believe the answer is:
stimulus
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'reaction litmus changes before its conclusion' is the wordplay.
'reaction' becomes 'u' (I am not sure about this - if you are sure you should believe this answer much more).
'changes' indicates anagramming the letters.
'before' means one lot of letters go next to another.
'conclusion' says to take the final letters.
The last letter of 'its' is 's'.
'litmus' is an anagram of 'stimul'.
'u' after 'stimul' is 'stimulu'.
'stimulu'+'s'='STIMULUS'
'produces' is the link.
Can you help me to learn more?
(Other definitions for stimulus that I've seen before include "Something that rouses to action" , "Something provoking a reaction" , "Something provoking a response" , "Incentive, provocation" , "Motivating action" .)