Letter from leaders of senate implicitly glorifying Mark Antony (5)
I believe the answer is:
sigma
'letter' is the definition.
(letter of the Greek alphabet)
'leaders of senate implicitly glorifying mark antony' is the wordplay.
'leaders of' indicates taking the first letters.
The initial letters of 'senate implicitly glorifying mark antony' is 'SIGMA'.
'from' acts as a link.
(Other definitions for sigma that I've seen before include "Gr. letter; rhymes with mark of disgrace" , "character abroad" , "Greek letter used as summation sign" , "How Socrates ended" , "Allowing no relaxation" .)