Adult finally in agreeable surroundings existing at birth (10)
I believe the answer is:
congenital
'existing at birth' is the definition.
(as in a congenital health condition)
'adult finally in agreeable' is the wordplay.
'adult finally' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'agreeable' becomes 'congenial' (synonyms).
't' put within 'congenial' is 'CONGENITAL'.
'surroundings' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
(Another definition for congenital that I've seen is " Absolute".)