False subject of litmus test over twelve month period (6)
I believe the answer is:
phoney
'false' is the definition.
(thesaurus)
'subject of litmus test over twelve month period' is the wordplay.
'subject of litmus test' becomes 'ph' (litmus test measures pH).
'over' becomes 'o' (cricket abbreviation).
'twelve month period' becomes 'ney' (I can't explain this - if you can you should believe this answer much more).
'ph'+'o'+'ney'='PHONEY'
(Other definitions for phoney that I've seen before include "Counterfeit" , "Fraud" , "Sham or insincere" , "Not genuine, insincere" , "Fraudulent" .)