Note that starts off the dawn chorus (3)
I believe the answer is:
sol
'note' is the definition.
(musical note as in do-re-mi-fa-sol-la-ti)
'starts off the dawn chorus' is the wordplay.
'off' indicates named letters should be taken away ('off' can mean to remove).
'the dawn' becomes 'start' (the dawn of something is the start of it).
'chorus' becomes 'ol' (I am not sure about this - if you are sure you should believe this answer much more).
'starts' with 'start' taken out is 's'.
's'+'ol'='SOL'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for sol that I've seen before include "The sun in Spain" , "Note; Peruvian currency" , "nearby star" , "Fifth note of scale" , "Spanish and poetic sun" .)