Live trial, one for insiders to counter (2,3)
I believe the answer is:
on air
'live' is the definition.
(as in broadcasting live on air)
'trial one for insiders to counter' is the wordplay.
'trial one' becomes 'rian' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'insiders' indicates the central letters.
'to counter' says the letters should be written backwards (counter can mean in the opposite direction).
The central letter of 'for' is 'o'.
'rian'+'o'='riano'
'riano' back-to-front is 'ON-AIR'.
Can you help me to learn more?
(Other definitions for on air that I've seen before include "Radio show might be" , "Broadcasting at this moment" , "In the process of broadcasting on radio or TV" , "(Walking) elatedly" , "very happily?" .)