A one-man club! (4)
I believe the answer is:
iron
'club' is the definition.
(iron is a kind of club)
'a one-man' is the wordplay.
'a 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 "stern" , "Strong metal; golf club" , "Heavy metallic element" , "Smooth; element" , "Metal obtained from haematite" .)