Basically, he's in a bitter environment (2,5)
I believe the answer is:
at heart
'basically' is the definition.
(both can mean fundamentally or deep down)
'he's in a bitter environment' is the wordplay.
'in' is an insertion indicator.
'bitter environment' becomes 'tart' (I can't justify this - if you can you should believe this answer much more).
'a'+'tart'='atart'
'he' placed inside 'atart' is 'AT HEART'.
(Other definitions for at heart that I've seen before include "In one's real nature - a threat (anag)" , "Substantially" , "real character" , "In reality" , "Fundamentally" .)