Third grade glossy turned over by graduate (5)
I believe the answer is:
gamma
'third grade' is the definition.
'gamma' can be an answer for 'grade' (I have seen 'Mediocre grade' mean 'gamma' so perhaps 'grade' could also mean 'gamma'). I'm not certain of the 'third' bit.
'glossy turned over by graduate' is the wordplay.
'glossy' becomes 'mag' (I've seen this in other clues).
'turned over' is a reversal indicator.
'by' says to put letters next to each other.
'graduate' becomes 'ma' (abbreviation for Master of Arts).
'mag' reversed gives 'gam'.
'gam'+'ma'='GAMMA'
Can you help me to learn more?
(Other definitions for gamma that I've seen before include "Greek letter G" , "A bit of Greek" , "Greek character" , "Poor mark" , "Greek letter - type of rays" .)