Initially hostile atmosphere that grows on one (4)
I believe the answer is:
hair
'that grows on one' is the definition.
(hair grows on one's body)
'initially hostile atmosphere' is the wordplay.
'initially' says to take the initial letters.
'atmosphere' becomes 'air' (synonyms).
The initial letter of 'hostile' is 'h'.
'h'+'air'='HAIR'
(Other definitions for hair that I've seen before include "Famous hippy musical of hirsute character" , "Showy garden flower" , "Fuzz" , "Nineteen sixties musical; filament" , "Fur, say" .)