Blithe or embarrassed about proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(I know that theorem is a type of proposition)
'blithe or embarrassed about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'THEOREM' is hidden within 'blithe or embarrassed'.
(Other definitions for theorem that I've seen before include "Logical proposition" , "(Maths) proposition" , "A rule in algebra" , "Statement to be proved in maths" , "Algebraic rule" .)