Relatively thick runner-up backs Conservative (6)
I believe the answer is:
closer
'relatively thick runner-up backs conservative' is the definition.
I can't tell whether this definition defines the answer.
'runner-up backs conservative' is the wordplay.
'runner-up' becomes 'loser' (runner-up can mean anyone other than the winner).
'backs' means one lot of letters go next to another.
'conservative' becomes 'C' (UK political abbreviation).
'loser' after 'c' is 'CLOSER'.
Can you help me to learn more?
(Other definitions for closer that I've seen before include "One who is skilled at bringing a business transaction to a satisfactory conclusion" , "Nearer or meaner" , "Nearer or more intimate" , "Nearer as to time or event" , "one concludes" .)