Bishop, during a trial, is taking the most optimistic view (2,4)
I believe the answer is:
at best
'taking the most optimistic view' is the definition.
(I've seen this before)
'bishop during a trial' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'during' means one lot of letters goes inside another.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'a'+'test'='atest'
'b' placed inside 'atest' is 'AT BEST'.
'is' is the link.
(Other definitions for at best that I've seen before include "In the most optimistic scenario" , "Under the optimum circumstances" , "Making every allowance" , "In the optimal situation" , "In the most favourable interpretation" .)