Teaching English, shelter dons (5)
I believe the answer is:
tenet
'teaching' is the definition.
(tenet is a kind of teaching)
'english shelter dons' is the wordplay.
'english' becomes 'e' (abbreviation).
'shelter' becomes 'tent' (tent is a kind of shelter).
'dons' means one lot of letters goes inside another (don means to wear or put on).
'e' going into 'tent' is 'TENET'.
(Other definitions for tenet that I've seen before include "Article of belief" , "A palindromic doctrine or belief" , "Doctrine; principle held" , "Belief held as true" , "That's the law" .)