In the most favourable scenario, a trial protects first born (2,4)
I believe the answer is:
at best
'in the most favourable scenario' is the definition.
I can't judge whether this defines the answer.
'a trial protects first born' is the wordplay.
'trial' becomes 'test' (synonyms).
'protects' is an insertion indicator (outer letters protect inside letters).
'first born' becomes 'b' (1st letter of 'born').
'test' enclosing 'b' is 'tbest'.
'a'+'tbest'='AT BEST'
Can you help me to learn more?
(Other definitions for at best that I've seen before include "In the optimum situation" , "In the most optimistic scenario" , "Taking the most hopeful view" , "Making every allowance" , "Under the optimum circumstances" .)