All so fine after splitting say (10)
I believe the answer is:
everything
'all' is the definition.
(all things)
'so fine after splitting say' is the wordplay.
'so' becomes 'very' (I've seen this in another clue).
'fine' becomes 'thin' (similar in meaning).
'after' means one lot of letters go next to another.
'splitting' means one lot of letters goes inside another.
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'very'+'thin'='verythin'
'verythin' put inside 'eg' is 'EVERYTHING'.
(Other definitions for everything that I've seen before include "great deal" , "The whole shebang" , "All items" , "All that is; life in general" , "the works" .)