Data seen in most unusual ecosystem (10)
I believe the answer is:
rainforest
'ecosystem' is the definition.
(rainforest is a type of ecosystem)
'data seen in most unusual' is the wordplay.
'data' becomes 'info' (information).
'seen in' is an insertion indicator.
'most unusual' becomes 'rarest' (rare can mean unusual or odd).
'info' going into 'rarest' is 'RAINFOREST'.
(Other definitions for rainforest that I've seen before include "Dense luxuriant tropical zone" , "area of ecological importance" , "geographical feature" , "Luxuriant expanse" , "high-growth area" .)