Disease caused by vitamin c deficiency (6)
I believe the answer is:
scurvy
'disease caused by vitamin c deficiency' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for scurvy that I've seen before include "Whoreson" , "Disease - epithet for knave" , "(Of a knave) contemptible!" , "Disease from lack of vitamin C - or dishonourable as knave may be" , "Condition caused by lack of vitamin C" .)