Unknown champ goes topless for nothing (4)
I believe the answer is:
zero
'nothing' is the definition.
(I know that nothing can be written as zero)
'unknown champ goes topless for' is the wordplay.
'unknown champ' becomes 'ze' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'goes topless' means to remove the first letter (I've seen 'going topless' mean this).
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'pro' with its first letter removed is 'ro'.
'ze'+'ro'='ZERO'
Can you help me to learn more?
(Other definitions for zero that I've seen before include "Nonentity" , "A very low temperature" , "Very bad visibility" , "Rock bottom" , "Loser" .)