Getting on after constant complaint (4)
I believe the answer is:
cold
'complaint' is the definition.
(cold is a kind of disease)
'getting on after constant' is the wordplay.
'getting on' becomes 'old' ('getting on a bit' means 'old').
'after' means one lot of letters go next to another.
'constant' becomes 'c' (mathematical abbreviation).
'old' put after 'c' is 'COLD'.
(Other definitions for cold that I've seen before include "Glacial" , "complaint" , "Common ailment" , "Unsympathetic" , "At low temperature" .)