Language teacher welcomed by greeting from Oz? (5)
I believe the answer is:
irish
'language' is the definition.
(Irish is an example)
'teacher welcomed by greeting from oz?' is the wordplay.
'teacher' becomes 'sir' (I've seen this before).
'welcomed by' means one lot of letters goes inside another (I've seen this in other clues).
'greeting' becomes 'hi' (hi is a kind of greeting).
'from oz?' shows that the letters should be reversed in order (upside down - like Australia is to the UK).
'sir' put into 'hi' is 'hsiri'.
'hsiri' back-to-front is 'IRISH'.
(Other definitions for irish that I've seen before include "From Dublin maybe" , "One setter" , "Nationality" , "Passion" , "From Galway, say" .)