The listener notes the rousing start (3)
I believe the answer is:
ear
'the listener' is the definition.
(body part which hears or listens)
'notes the rousing start' is the wordplay.
'notes' becomes 'ea' ().
'start' indicates taking the first letters.
The initial letter of 'rousing' is 'r'.
'ea'+'r'='EAR'
(Other definitions for ear that I've seen before include "Aural organ" , "Heed" , "one hears" , "Ability to recognise sounds" , "Fruiting head of a cereal" .)