One getting letters of distress to stay away interests the geologist, whose crust is in the balance (8)
I believe the answer is:
isostasy
'balance' is the definition.
(I have seen 'earthly balance' mean 'isostasy' so perhaps 'balance' could also mean 'isostasy')
'one getting letters of distress to stay away' is the wordplay.
'one' becomes 'i' (Roman numeral).
'getting' means one lot of letters go next to another.
'letters of distress' becomes 'sos' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to' says to put letters next to each other.
'away' is an anagram indicator.
'stay' with letters rearranged gives 'tasy'.
'i'+'sos'+'tasy'='ISOSTASY'
'interests the geologist whose crust is in the' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for isostasy that I've seen before include "Geological equilibrium" , "earthly balance" .)