Flatten with a club (4)
I believe the answer is:
iron
I believe this clue is a double definition.
'flatten' is the first definition.
(I've seen this before)
'a club' is the second definition.
(iron is a kind of club)
'with' acts as a link.
(Other definitions for iron that I've seen before include "Club; element" , "Flatten; element" , "stern" , "Heavy metallic element" , "A magnet attracts it" .)