Signaller's first to finish transmission (7)
I believe the answer is:
sending
'transmission' is the definition.
(thesaurus)
'signaller's first to finish' is the wordplay.
'signaller's first' becomes 's' (1st letter of 'signaller').
'to finish' becomes 'ending' (I've seen this before).
's'+'ending'='SENDING'
(Other definitions for sending that I've seen before include "Despatching" , "Transmitting (a message)" , "Dispatching, transmitting" .)