He wrote notes alongside river delta (4)
I believe the answer is:
byrd
'he wrote notes' is the definition.
The definition suggests a singular noun which matches the answer.
'alongside river delta' is the wordplay.
'alongside' becomes 'by' (I've seen this in other clues).
'river' becomes 'r'.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'by'+'r'+'d'='BYRD'
Can you help me to learn more?
(Other definitions for byrd that I've seen before include "Prolific scorer" , "English composer" , "Elizabethan composer; sounds like avian" , "polar explorer" , "William ---, Tudor composer" .)