Red cell deficiency (7)
I believe the answer is:
anaemia
'red cell deficiency' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for anaemia that I've seen before include "Lack of red cells" , "Red blood cell deficiency" , "Disorder of deficiency in red blood cells" , "Blood deficiency condition" , "Effect of bad blood" .)