Over a spleen infection (7)
I believe the answer is:
cholera
'infection' is the definition.
(I know that cholera is a infection)
'over a spleen' is the wordplay.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'spleen' becomes 'choler' (choler is a kind of spleen).
'a' put after 'choler' is 'CHOLERA'.
(Other definitions for cholera that I've seen before include "Deadly, infectious disease" , "Infectious and often fatal Asiatic disease" , "Oh, Clare has highly infectious disease" , "Solemn promise" , "Disease caught from contaminated water" .)