Flood at home, before unknown disease (6)
I believe the answer is:
influx
'flood' is the definition.
(I've seen this before)
'home before unknown disease' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'before' means one lot of letters go next to another.
'unknown' becomes 'x' (algebra).
'disease' becomes 'flu' (flu is a kind of health problem).
'x' after 'flu' is 'flux'.
'in'+'flux'='INFLUX'
'at' is the link.
(Other definitions for influx that I've seen before include "Flood coming from outside" , "Inundation" , "Mass intrusion" , "Process of arrival" , "Arrival, flood" .)