This crossword setter tore off — a streaker? (6)
I believe the answer is:
meteor
'a streaker?' is the definition.
(something which streaks across the sky)
'this crossword setter tore off' is the wordplay.
'this crossword setter' becomes 'me' (referring to the person writing the clues).
'off' indicates anagramming the letters.
'tore' anagrammed gives 'teor'.
'me'+'teor'='METEOR'
(Other definitions for meteor that I've seen before include "Component of shower" , "Small extraterrestrial body" , "Falling star" , "It causes a streak of light in the night sky" , "Remote sort of heavenly body" .)