Symptom of a cold (6)
'symptom of a cold' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sneeze that I've seen before include "Expel air from nose" , "Explosive nasal action" , "Exhale violently" , "Involuntary expulsion of air from the nose" , "Explosion of air through nose and mouth" .)
(Other definitions for catarrh that I've seen before include "Inflammation of mucous membrane with watery discharge" , "Nasal problem" , "cold symptom" , "Problem with cold" , "Nasal discharge of mucus" .)