One objecting to 50% reduction in proper sample (9)
I believe the answer is:
protester
'one objecting' is the definition.
(a protestor objects to something)
'proper sample' is the wordplay.
'proper' becomes 'pro' (I can't justify this - if you can you should believe this answer much more).
'sample' becomes 'tester' (I've seen this before).
'pro'+'tester'='PROTESTER'
'to 50 reduction in' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for protester that I've seen before include "Dissident" , "Person, we hear, not content" , "objector" , "I strongly object" , "Demonstrator" .)