A tonic in the club (4)
I believe the answer is:
iron
'club' is the definition.
(type of golf club)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for iron that I've seen before include "Smooth - metal" , "A magnet attracts it" , "Decrease" , "Metallic element, symbol Fe" , "great strength" .)