O, a billet-doux! (4,6)
I believe the answer is:
love letter
'billet-doux' is the definition.
(I've seen this before)
'o a' is the wordplay.
I cannot quite see how this works, but
'o' could be 'love' (I've seen this in other clues) and 'love' is present in the answer.
The remaining letters 'letter' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for love letter that I've seen before include "caring note!" , "Billet-doux" , "romantic reading material" .)