Disease beginning to spread, harmless in seaweed (5,5)
I believe the answer is:
lassa fever
'disease' is the definition.
(I've seen this before)
'beginning to spread harmless in seaweed' is the wordplay.
'beginning to' indicates taking the first letters.
'harmless' becomes 'safe' ('safe' is associated in meaning with 'harmless').
'in' is an insertion indicator.
'seaweed' becomes 'laver' (I have seen 'Edible seaweed ' mean 'laver' so perhaps 'seaweed' could also mean 'laver'**).
The first letter of 'spread' is 's'.
's'+'safe'='ssafe'
'ssafe' placed inside 'laver' is 'LASSA FEVER'.
(Other definitions for lassa fever that I've seen before include "Tropical viral disease that could kill" , "Acute viral disease" , "ailment" .)