Oscar in charge in case? That's best! (8)
I believe the answer is:
choicest
'best' is the definition.
(I've seen this before)
'oscar in charge in case?' is the wordplay.
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'in charge' becomes 'ic'.
'in' is an insertion indicator.
'case?' becomes 'chest' (**).
'o'+'ic'='oic'
'oic' inserted into 'chest' is 'CHOICEST'.
'that's' acts as a link.
(Other definitions for choicest that I've seen before include "Best" , "Top" .)