What does the cryptic crossword clue "Screening thirds, this examiner does lines from Sonnets (7)" mean?

The wordplay uses a deletion. SES (deletion) + TETS (deletion) = SESTETS The answer is SESTETS.

Screening thirds, this examiner does lines from Sonnets (7)

Guardian Cryptic #29521 | 25 Across | 2024-10-23 flower