A one-man club! (4)
I believe the answer is:
'club' is the definition.
(type of golf club)
'a one-man' is the wordplay.
'a one' becomes 'I' (Roman numeral).
'man' becomes 'ron' (short for Ronald).
(Other definitions for iron that I've seen before include "great strength", "Former dividing line across Europe", "What's used to make pitch", "rations in emergency?", "Club to smooth the cloth".)