A Greek character upset, very unwell and ineffectual (10)
I believe the answer is:
unavailing
'ineffectual' is the definition.
(synonyms)
'a greek character upset very unwell' is the wordplay.
'greek character' becomes 'nu' (letter from the Greek alphabet).
'upset' says the letters should be written backwards.
'very' becomes 'v' (common abbreviation).
'unwell' becomes 'ailing' (synonyms).
'a'+'nu'='anu'
'anu' written backwards gives 'una'.
'una'+'v'+'ailing'='UNAVAILING'
'and' acts as a link.
(Another definition for unavailing that I've seen is " Achieving little or nothing".)