Rich one, Conservative joining club (6)
I believe the answer is:
ironic
'rich' is the definition.
(I've seen this before)
'one conservative joining club' is the wordplay.
'one' becomes 'i' (Roman numeral).
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'joining' is a charade indicator (letters next to each other).
'club' becomes 'iron' (iron is a kind of club**).
'i'+'c'='ic'
'ic' after 'iron' is 'IRONIC'.
(Other definitions for ironic that I've seen before include "Wryly funny" , "Humorously sarcastic or mocking" , "These words express the opposite of the literal meaning" , "Wryly humorous" , "Mockingly humorous" .)