Please, teacher, I have a letter (7)
I believe the answer is:
missive
'a letter' is the definition.
(I know that letter can be written as missive)
'please teacher i have' is the wordplay.
'please teacher' becomes 'miss' (I can't justify this - if you can you should give a lot more credence to this answer).
'i have' becomes 'I've' (contraction of 'I have').
'miss'+'ive'='MISSIVE'
Can you help me to learn more?
(Other definitions for missive that I've seen before include "message" , "intelligence in this?" , "Letter" .)