In the morning, grunts lifting up letter (5)
I believe the answer is:
sigma
'letter' is the definition.
(sigma is a kind of letter)
'in the morning grunts lifting up' is the wordplay.
'in the morning' becomes 'am'.
'grunts' becomes 'gis' (I can't explain this - if you can you should believe this answer much more).
'lifting up' is a reversal indicator (I've seen 'lift up' mean this).
'am'+'gis'='amgis'
'amgis' reversed gives 'SIGMA'.
Can you help me to learn more?
(Other definitions for sigma that I've seen before include "character abroad" , "Gr. letter; rhymes with mark of disgrace" , "Allowing no relaxation" , "letter to Greeks" , "Greek letter corresponding to S" .)