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