When you've lost, possibly out for a duck, here's a supporter (5)
I believe the answer is:
stool
'a supporter' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'when you've lost possibly out for a duck' is the wordplay.
I cannot quite see how this works, but
'lost' could be 'l' (abbreviation in sporting league tables) and 'l' is found within the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is found within the answer.
'duck' could be 'o' (resembles zero - 'duck' in cricket) and 'o' is found in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'here's' acts as a link.
Can you help me to learn more?
(Other definitions for stool that I've seen before include "Motion" , "place butt on one?" , "Seat without a back" , "Backless chair" , "item of furniture here" .)