I could be the merry sonnet’s source (9)
I believe the answer is:
rhymester
'i' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?
'could be the merry sonnet's source' is the wordplay.
'could be' indicates anagramming the letters.
'source' suggests taking the first letters (I've seen 'source of' mean this).
The first letter of 'sonnets' is 's'.
'the'+'merry'+'s'='themerrys'
'themerrys' is an anagram of 'RHYMESTER'.
(Other definitions for rhymester that I've seen before include "Would-be poet" , "Inferior poet" , "Poor poet" , "Versifier" , "mediocre poet" .)