Red blood cell deficiency (7)
I believe the answer is:
anaemia
'red blood 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 "Result of iron deficiency" , "complaint of blood" , "Red blood cell deficiency" , "Blood disorder -- ie I am Ana (anag)" , "cause of pallor" .)