Pretty ducks are a legitimate target (4,4)
I believe the answer is:
fair game
'a legitimate target' is the definition.
(I've seen this in another clue)
'pretty ducks' is the wordplay.
'pretty' becomes 'fair' (both can mean beautiful or attractive).
'ducks' becomes 'game' (I can't justify this - if you can you should give a lot more credence to this answer).
'fair'+'game'='FAIR GAME'
'are' is the link.
Can you help me to learn more?
(Other definitions for fair game that I've seen before include "can be justly criticised" , "person open to criticism" , "Aunt Sally, for example" , "Acceptable target?" , "coconut shy?" .)