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 tell whether this defines the answer.
'at poker try not to do so' is the wordplay.
'at' means one lot of letters go next to another.
'poker try' becomes 'fl' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'not' indicates named letters should be taken away.
'do' becomes 'ut' (both can mean the first note in a musical scale).
'so' becomes 'soh' (synonyms).
'ut'+'soh'='utsoh'
'utsoh' with 'to' removed is 'ush'.
'fl'+'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" .)