The setter's back story prior to university environment (6)
I believe the answer is:
milieu
'environment' is the definition.
(milieu is a kind of environment)
'the setter's back story prior to university' is the wordplay.
'the setter's' becomes 'im' (referring to the person who set the crossword).
'back' is a reversal indicator.
'story' becomes 'lie' (story can mean a fabrication).
'prior to' is a charade indicator (letters next to each other).
'university' becomes 'u'.
'im' written backwards gives 'mi'.
'mi'+'lie'+'u'='MILIEU'
(Other definitions for milieu that I've seen before include "Environment - setting" , "Environmental condition, surrounding" , "'Surroundings, setting (6)'" , "(Social) setting" , "Social environment or set of surroundings" .)