At poker, try not to do so when you have one! (5)
I believe the answer is:
flush
'you have one' is the definition.
I can't judge whether this defines the answer.
'at poker try not to do so' is the wordplay.
'at' is a charade indicator (letters next to each other).
'poker try' becomes 'fl' (I can't explain this - if you can you should believe this answer much more).
'not' is a deletion indicator.
'do' becomes 'ut' (both can mean the first note in a musical scale).
'so' becomes 'soh' (synonyms).
'ut'+'soh'='utsoh'
'utsoh' with 'to' taken out is 'ush'.
'fl' next to 'ush' is 'FLUSH'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for flush that I've seen before include "with a load of cash" , "winning hand at poker" , "Become red and hot" , "Rich" , "Rouse game and put to flight" .)