Alerting but not if you start in first gear (9)
I believe the answer is:
notifying
'alerting' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'not if you start in first gear' is the wordplay.
'you start' becomes 'y' (I've seen this in other clues).
'first gear' becomes 'g' (1st letter of 'gear').
'not'+'if'+'y'+'in'+'g'='NOTIFYING'
'but' acts as a link.
Can you help me to learn more?
(Another definition for notifying that I've seen is " Officially informing (someone)".)