Top-class teacher having a lie-in? (4)
I believe the answer is:
abed
'having a lie-in?' is the definition.
(abed can mean in bed)
'top-class teacher' is the wordplay.
'top class' becomes 'a' (top grade).
'teacher' becomes 'BEd' (Bachelor of Education).
'a'+'bed'='ABED'
(Other definitions for abed that I've seen before include "at rest" , "going to sleep?" , "laid up" , "Having not got up" , "dream location?" .)