Flag in tennis match revealing varying set of principles (6,9)
I believe the answer is:
double standards
'principles' is the definition.
(I have seen 'conflicting principles' mean 'double standards' so perhaps 'principles' could also mean 'double standards')
'flag in tennis' is the wordplay.
'flag' becomes 'standard' (standard is a kind of flag).
'in' means one lot of letters goes inside another.
'tennis' becomes 'doubles' (doubles is a kind of tennis).
'standard' inserted into 'doubles' is 'DOUBLE STANDARDS'.
'match revealing varying set of' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for double standards that I've seen before include "Put out more flags" , "showing inconsistent principles" , "Blue Peter and Jolly Roger may express" , "conflicting principles" , "hypocrisy" .)