Ruin agent set up by subordinate (8)

'ruin' is the definition.
(I know that ruin can be written as deflower)

'agent set up by subordinate' is the wordplay.
'agent' becomes 'fed' (fed is a kind of agent).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'by' says to put letters next to each other.
'subordinate' becomes 'lower' (synonyms).
'fed' written backwards gives 'def'.
'def'+'lower'='DEFLOWER'