Deficiency of red blood cells (7)
I believe the answer is:
anaemia
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deficiency of red blood cells</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" , "Lack of red cells" , "Pallid condition" , "Pale condition" , "Disorder of deficiency in red blood cells" .)