Blood disorder (7)
I believe the answer is:
anaemia
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">blood disorder</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 "Lack of red corpuscles in blood resulting in weakness" , "Blood deficiency condition" , "Pallid condition" , "Red blood cell deficiency" , "Bloodlessness" .)