Twain boy most upset by solicitor dropping line (3,6)
I believe the answer is:
tom sawyer
'twain boy' is the definition.
(character created by Mark Twain)
'most upset by solicitor dropping line' is the wordplay.
'upset by' indicates an anagram.
'solicitor' becomes 'lawyer' (I've seen this before).
'dropping' indicates named letters should be taken away.
'line' becomes 'l'.
'lawyer' with 'l' taken out is 'awyer'.
'most' anagrammed gives 'toms'.
'toms'+'awyer'='TOM SAWYER'
(Other definitions for tom sawyer that I've seen before include "Hero of book" , "Mark Twain character" , "Mark Twain's adventurous boy" , "book's hero" , "Boy whose Adventures were recorded by Mark Twain" .)