Greek author upset water on work (5)
I believe the answer is:
aesop
'greek author' is the definition.
(I've seen this before)
'upset water on work' is the wordplay.
'upset' says the letters should be written in reverse.
'water' becomes 'sea' (sea is a kind of water).
'on' means one lot of letters go next to another.
'work' becomes 'op' (abbreviation for opus).
'sea' reversed gives 'aes'.
'aes'+'op'='AESOP'
(Other definitions for aesop that I've seen before include "So ape the writer of fables" , "Author of many fables" , "his writings were fabulous" , "satirical writer" , "fabulous figure?" .)