Skinnier student with empty stomach (6)
I believe the answer is:
leaner
'skinnier' is the definition.
('lean' can be similar in meaning to 'skinny')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for leaner that I've seen before include "less productive" , "One with an inclination" , "Thinner, with less fat" , "less corpulent?" , "less bulky" .)