Red blood cell deficiency (7)
I believe the answer is:
anaemia
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">red blood cell deficiency</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)