One in distance beginning to upset environment (6)
I believe the answer is:
milieu
'environment' is the definition.
(I know that milieu is a type of environment)
'one in distance beginning to upset' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'distance' becomes 'mile' ().
'beginning to' says to take the initial letters.
The first letter of 'upset' is 'u'.
'i' put into 'mile' is 'milie'.
'milie'+'u'='MILIEU'
(Other definitions for milieu that I've seen before include "Surroundings - location" , "'Surroundings, setting (6)'" , "(Social) setting" , "Environmental condition, surrounding" , "Environment - setting" .)