Light shade contains a blemish (5)
I believe the answer is:
taint
'blemish' is the definition.
(I've seen this before)
'light shade contains a' is the wordplay.
'light shade' becomes 'tint' (I've seen this in another clue).
'contains' is an insertion indicator.
'tint' going around 'a' is 'TAINT'.
(Other definitions for taint that I've seen before include "Flaw in Titan" , "Stigmatise" , "Blemish" , "Contaminating influence" , "Source of infection" .)