Cad knocked over, achieve objective (6)
I believe the answer is:
target
'objective' is the definition.
(I know that objective can be written as target)
'cad knocked over achieve' is the wordplay.
'cad' becomes 'rat' (I've seen this before).
'knocked over' says the letters should be written backwards.
'achieve' becomes 'get' (I've seen this in another clue).
'rat' backwards is 'tar'.
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "'Aim, goal (6)'" , "Goal to be aimed for" , "A result to aim at" , "Objective, mark" , "Butt" .)