Lecturer tucked into square meal (5)
I believe the answer is:
flour
'meal' is the definition.
(I've seen this before)
'lecturer tucked into square' is the wordplay.
'lecturer' becomes 'l'.
'tucked into' means one lot of letters goes inside another.
'square' becomes 'four' (four is a square number**).
'l' placed within 'four' is 'FLOUR'.
(Other definitions for flour that I've seen before include "Finely ground meal" , "Fine-ground grain" , "Wheat powder" , "Delivery from miller" , "Main ingredient of bread" .)