Rum ration left as a test (2,5)
I believe the answer is:
on trial
'as a test' is the definition.
'rum ration left' is the wordplay.
'rum' indicates anagramming the letters (rum can mean strange or unusual).
'left' becomes 'l' (common abbreviation).
'ration' with letters rearranged gives 'ontria'.
'ontria'+'l'='ON TRIAL'
(Other definitions for on trial that I've seen before include "like a footballer seeking club?" , "Probationary" , "being judged" , "In court to answer charges" , "Undergoing scrutiny" .)