Teacher sounds a bit uncertain (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'sounds a bit uncertain' is the wordplay.
'a bit' indicates taking the first letters (I've seen 'bit of' mean this).
'uncertain' becomes 'ir' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letter of 'sounds' is 's'.
's'+'ir'='SIR'
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Term of address for a gentleman" , "Male title of respect" , "Form of address for a knight" , "Formal and deferential address to a man" , "Knight's title" .)