Ceasing transmitting — not the first (6)
I believe the answer is:
ending
'ceasing' is the definition.
('end' can be a synonym of 'cease')
'transmitting not the first' is the wordplay.
'transmitting' becomes 'sending' ('send' can be a synonym of 'transmit').
'not the first' means to remove the first letter.
'sending' with its initial letter removed is 'ENDING'.
(Other definitions for ending that I've seen before include "Finishing" , "Cessation" , "Climax" , "Finale" , "Coda" .)