Frontier as found in tropical woods (10)
I believe the answer is:
rainforest
'frontier as found in' is the definition.
The definition suggests a singular noun which matches the answer.
'tropical woods' is the wordplay.
'tropical' becomes 'rain' (I can't justify this - if you can you should give a lot more credence to this answer).
'woods' becomes 'forest' (synonyms).
'rain'+'forest'='RAINFOREST'
Can you help me to learn more?
(Other definitions for rainforest that I've seen before include "has lots of tropical trees" , "Tropical jungle" , "Dense luxuriant tropical zone" , "high-growth area" , "geographical feature" .)