50% off sparkling wines - that's a winner! (5)
I believe the answer is:
champ
'winner' is the definition.
(I know that champ is a type of winner)
'50% off sparkling wines that's a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'amp' (A is the symbol for the amp or ampere) and 'amp' is found in the answer.
The remaining letters 'ch' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for champ that I've seen before include "Bite - victor (abbr)" , "Mashed potato with scallions" , "The first person" , "Munch (at the bit?)" , "boxer who is top dog?" .)