Gentle soul, getting US lawyer for character from Athens (6)
I believe the answer is:
lambda
'character from athens' is the definition.
(letter from the Greek alphabet)
'gentle soul getting us lawyer' is the wordplay.
'gentle soul' becomes 'lamb' (I've seen this in another clue).
'getting' says to put letters next to each other.
'us lawyer' becomes 'DA' (abbreviation for District Attorney).
'lamb'+'da'='LAMBDA'
'for' acts as a link.
(Other definitions for lambda that I've seen before include "Greek letter L" , "Greek letter - am bald (anag)" , "Foreign character" , "Not one of our characters" , "character from Greece" .)