Being defiant is unacceptable in a casual relationship (8)
I believe the answer is:
flouting
'being defiant is' is the definition.
I can't judge whether this defines the answer.
'unacceptable in a casual relationship' is the wordplay.
'unacceptable' becomes 'out' (out can mean incorrect or inappropriate).
'in' means one lot of letters goes inside another.
'a casual relationship' becomes 'fling' (I have seen 'Brief relationship' mean 'fling' so perhaps 'relationship' could also mean 'fling'. I am not sure about the 'casual' bit.).
'out' placed within 'fling' is 'FLOUTING'.
Can you help me to learn more?
(Other definitions for flouting that I've seen before include "Disobeying; breaking (rule)" , "ridicule" , "Disregarding with contempt" .)