Sniffles caught by elderly (4)
I believe the answer is:
cold
'sniffles' is the definition.
(thesaurus)
'caught by elderly' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'by' is a charade indicator (letters next to each other).
'elderly' becomes 'old' (similar in meaning).
'c'+'old'='COLD'
(Other definitions for cold that I've seen before include "At low temperature" , "Lacking affection" , "Common ailment" , "complaint" , "Chill" .)