Some feckless Irish teacher (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'some feckless irish' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SIR' is hidden in the letters of 'feckless irish'.
(Other definitions for sir that I've seen before include "Form of address for a knight" , "Polite or formal address to male" , "Polite form of address to a man" , "Term of address for a gentleman" , "Polite term of address for a man" .)