Pronoun the writer felt out of place, penning start of sonnet (6)
I believe the answer is:
itself
'pronoun' is the definition.
(I've seen this in another clue)
'writer felt out of place penning start of sonnet' is the wordplay.
'writer' becomes 'I' (the person who is writing these clues).
'out of place' indicates an anagram.
'penning' indicates putting letters inside.
'start of' says to take the initial letters.
The first letter of 'sonnet' is 's'.
'felt' anagrammed gives 'telf'.
'telf' going around 's' is 'tself'.
'i'+'tself'='ITSELF'
'the' is the link.
(Other definitions for itself that I've seen before include "Emphatic form of it" , "That's it, emphatically" , "Reflexive word - stifle (anag)" , "Third-person reflexive pronoun" , "The thing, I stress" .)