One-man club (4)
I believe the answer is:
iron
'club' is the definition.
(iron is a kind of club)
'one-man' is the wordplay.
'one' becomes 'i' (Roman numeral).
'man' becomes 'ron' (I've seen this before**).
'i'+'ron'='IRON'
(Other definitions for iron that I've seen before include "Main element in steel" , "Revolver" , "Main component of steel" , "Very robust" , "Metal (that may be wrought)" .)