Bowled, during a trial, under the most favourable conditions (2,4)
I believe the answer is:
at best
'under the most favourable conditions' is the definition.
(I've seen this before)
'bowled during a trial' is the wordplay.
'bowled' becomes 'b' (cricket abbreviation).
'during' is an insertion indicator.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'a'+'test'='atest'
'b' inserted into 'atest' is 'AT BEST'.
(Other definitions for at best that I've seen before include "Under optimum conditions" , "Making every allowance" , "Taking the most hopeful view" , "Under the optimum circumstances" , "In the most optimistic scenario" .)