Deficiency of red blood cells (7)
I believe the answer is:
anaemia
'deficiency of red blood cells' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for anaemia that I've seen before include "Result of iron deficiency" , "Lack of red cells" , "Pallid condition" , "Pale condition" , "Disorder of deficiency in red blood cells" .)