Streak of light encountered upset small deer (6)
I believe the answer is:
meteor
'streak of light' is the definition.
(meteors streak through the sky)
'encountered upset small deer' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'upset' says the letters should be written in reverse.
'small deer' becomes 'roe' (I've seen this in another clue).
'roe' written backwards gives 'eor'.
'met'+'eor'='METEOR'
(Other definitions for meteor that I've seen before include "Shooting star" , "extraterrestrial debris" , "Small extraterrestrial body" , "lump of rock" , "Small piece of matter in the solar system" .)