Since you lose when you win, it's a tricky thing! (6)
I believe the answer is:
misere
'you lose when you win it's a tricky thing' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'since' is the wordplay.
I cannot quite understand how this works, but
'since' could be 'ere' (I've seen this in another clue) and 'ere' is located in the answer.
The remaining letters 'mis' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for misere that I've seen before include "undertaking to take no tricks" , "Taking no tricks" , "Solo whist no-tricks bid" , "Card-player's call" .)