A lot of people I originally found in a mountainous region (6)
I believe the answer is:
massif
'a mountainous region' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a lot of people i originally found' is the wordplay.
'a lot of people' becomes 'mass' (mass of people).
'originally' suggests taking the first letters.
The initial letter of 'found' is 'f'.
'mass'+'i'+'f'='MASSIF'
'in' is the link.
Can you help me to learn more?
(Other definitions for massif that I've seen before include "Geological mountain formation" , "Extensive raised area" , "Mountainous block" , "High points" , "Compact mountainous area" .)