Conservative with round middle is source of jokes (8)
I believe the answer is:
comedian
'source of jokes' is the definition.
'conservative with round middle' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'with round' becomes 'o' (round can mean a round object. I am not sure about the 'with' bit.**).
'middle' becomes 'median'.
'c'+'o'+'median'='COMEDIAN'
'is' is the link.
(Other definitions for comedian that I've seen before include "Funny entertainer" , "Hope, perhaps" , "I'm not taken seriously" , "Cracker?" , "scream?" .)