Rarest of indigenous creatures originally in ark? (8)
I believe the answer is:
choicest
'rarest' is the definition.
'of indigenous creatures originally in ark?' is the wordplay.
'originally' indicates taking the first letters.
'in' indicates putting letters inside.
'ark?' becomes 'chest' (ark is a kind of chest**).
The initial letters of 'of indigenous creatures' is 'oic'.
'oic' placed inside 'chest' is 'CHOICEST'.
(Other definitions for choicest that I've seen before include "Best" , "Top" .)