Morning publication returned letter (5)
I believe the answer is:
gamma
'letter' is the definition.
(gamma is a kind of letter of the alphabet)
'morning publication returned' is the wordplay.
'morning' becomes 'am'.
'publication' becomes 'mag' (abbreviation for magazine).
'returned' says the letters should be written backwards.
'am'+'mag'='ammag'
'ammag' backwards is 'GAMMA'.
(Other definitions for gamma that I've seen before include "Mediocre grade" , "Greek letter - type of rays" , "Poor mark" , "third highest grade" , "Third letter of the Greek alphabet" .)