Frank seeing that there's a final note (7)
I believe the answer is:
sincere
'frank' is the definition.
(frank can mean sincere or honest)
'seeing that there's a final note' is the wordplay.
I cannot quite see how this works, but
'seeing' could be 'since' (I've seen this before) and 'since' is found within the answer.
'note' could be 're' (as in do-re-mi) and 're' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sincere that I've seen before include "like informal correspondents?" , "Honest, frank" , "Showing genuine feelings" , "Earnest" , "Pure" .)