First woman to enter students' flat! (5)
I believe the answer is:
level
'flat' is the definition.
(I know that flat can be written as level)
'first woman to enter students'' is the wordplay.
'first woman' becomes 'Eve' (first woman in the bible).
'to enter' means one lot of letters goes inside another.
'students'' becomes 'LL' (two abbreviations for learner).
'eve' going inside 'll' is 'LEVEL'.
(Other definitions for level that I've seen before include "Regular" , "Straight, even" , "Flat and smooth" , "On equal terms" , "Horizontal line" .)