Vitamin C deficiency (6)
I believe the answer is:
scurvy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vitamin c 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 scurvy that I've seen before include "(Of a knave) contemptible!" , "Disease once associated with poorly-nourished sailors" , "problem for many a sailor?" , "Disease caused by lack of vitamin C" , "an old naval problem" .)