Equestrian costumes or customs (6,6)
I believe the answer is:
riding habits
'equestrian costumes' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps you can see a link between them that I don't see?
'customs' is the wordplay.
I cannot really see how this works, but
'customs' could be 'habits' (habit is a kind of custom) and 'habits' is found within the answer.
The remaining letters 'riding' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'or' acts as a link.
Can you help me to learn more?
(Another definition for riding habits that I've seen is " suit the mounted".)