Part of year depicted in end of sonnet by old poet (9)
I believe the answer is:
trimester
'part of year' is the definition.
'trimester' can be an answer for 'year' (thesaurus). I'm unsure of the 'part of' bit.
'depicted in end of sonnet by old poet' is the wordplay.
I cannot really see how this works, but
'poet' could be 'rimester' (alternative spelling of rhymester) and 'rimester' is found within the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trimester that I've seen before include "US academic term" , "academic session" , "Period of three months" , "part of college year" , "American term" .)