Flirtation marked a sick turn in disco, say (9)
I believe the answer is:
dalliance
'flirtation' is the definition.
(both can mean a casual relationship)
'a sick turn in disco say' is the wordplay.
'sick' becomes 'ill' (synonyms).
'turn' says the letters should be written backwards.
'in' indicates putting letters inside.
'disco say' becomes 'dance' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'ill'='aill'
'aill' written backwards gives 'llia'.
'llia' put within 'dance' is 'DALLIANCE'.
'marked' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dalliance that I've seen before include "waste of time" , "Dawdling" , "Affair" , "Casual relationship" , "Fling" .)