Scheme corporation has to employ student (4)
I believe the answer is:
plot
'scheme' is the definition.
(plot is a kind of scheme)
'corporation has to employ student' is the wordplay.
'corporation' becomes 'pot' (corporation can humorously mean a pot belly).
'has to employ' means one lot of letters goes inside another (inserted letters are brought in or employed).
'student' becomes 'l' (as in L-plates for learner drivers).
'pot' going around 'l' is 'PLOT'.
(Other definitions for plot that I've seen before include "Graph; piece of ground" , "Vegetable garden - scheme" , "Secret scheme to do something underhand" , "bed" , "Conspire - small piece of land" .)