Rob him, the fathead! (3)
I believe the answer is:
mug
'rob' is the definition.
(slang)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mug that I've seen before include "Tankard" , "Slangy victim, by gum" , "A drinking vesel or a slangy victim" , "Vessel for a bit of a fool" , "Cup; stupid person" .)