Shockingly vivid sources of light under grid no good (5)
I believe the answer is:
lurid
'shockingly vivid' is the definition.
(I've seen this before)
'sources of light under grid no good' is the wordplay.
'sources of light' becomes 'lu' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'under' says to put letters next to each other.
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'grid' with 'g' taken out is 'rid'.
'lu'+'rid'='LURID'
(Other definitions for lurid that I've seen before include "Glaringly vivid and graphic" , "Fiery - sensational" , "Gloomily threatening" , "Glaringly bright" , "In vivid detail" .)