Frontier as worryingly endangered area (10)
I believe the answer is:
rainforest
'endangered area' is the definition.
The definition suggests a singular noun which matches the answer.
'frontier as worryingly' is the wordplay.
'worryingly' indicates anagramming the letters (letters are disturbed or worried).
'frontier'+'as'='frontieras'
'frontieras' anagrammed gives 'RAINFOREST'.
Can you help me to learn more?
(Other definitions for rainforest that I've seen before include "Dense luxuriant tropical zone" , "Tropical jungle" , "geographical feature" , "high-growth area" , "Luxuriant tropical woodland" .)