Left most of Essex with liver upset — worse things can happen (6,4)

'worse things can happen' is the definition.
(the lesser evil is not the worst possibility)

'left most of essex with liver upset' is the wordplay.
'left' becomes 'l' (common abbreviation).
'most of' means to remove the last letter.
'with' is a charade indicator (letters next to each other).
'upset' says the letters should be written in reverse.
'liver' backwards is 'revil'.
'essex' with its final letter taken away is 'esse'.
'l'+'esse'+'revil'='LESSER EVIL'