It's odd, yet mostly the opposite (5)
I believe the answer is:
seven
'opposite' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'it's odd yet mostly' is the wordplay.
'it's odd' becomes 'se' (I can't explain this - if you can you should give a lot more credence to this answer).
'yet' becomes 'even' (synonyms).
'mostly' means to remove the first letter.
'even' with its first letter taken off is 'ven'.
'se'+'ven'='SEVEN'
'the' is the link.
Can you help me to learn more?
(Other definitions for seven that I've seen before include "Film" , "Number of sisters in Sussex" , "Number of colours of the rainbow" , "Prime number" , "- Against Thebes (Aeschylus)" .)