Sickness and diarrhoea at first is moderate (9)
I believe the answer is:
distemper
'sickness' is the definition.
(distemper is a kind of sickness)
'diarrhoea at first is moderate' is the wordplay.
'at first' says to take the initial letters.
'moderate' becomes 'temper' ('temper' can be a synonym of 'moderate').
The first letter of 'diarrhoea' is 'd'.
'd'+'is'+'temper'='DISTEMPER'
'and' is the link.
(Other definitions for distemper that I've seen before include "Water-based paint" , "Paint - dog disease" , "Infectious disease of animals" , "Sickness" , "Viral infection < found in lab" .)