Language teacher has again impressed beginners (4)
I believe the answer is:
thai
'language' is the definition.
(Thai is an example)
'teacher has again impressed beginners' is the wordplay.
'beginners' indicates taking the first letters.
The initial letters of 'teacher has again impressed' is 'THAI'.
(Other definitions for thai that I've seen before include "Sort of massage" , "nationality" , "Easterner" , "Native of Southeast Asia" , "From Bangkok, for example" .)