Perish swallowing salt water with sulphur, a possible cause of death (7)
I believe the answer is:
disease
'a possible cause of death' is the definition.
(a disease can be fatal)
'perish swallowing salt water with sulphur' is the wordplay.
'perish' becomes 'die' (synonyms).
'swallowing' means one lot of letters goes inside another.
'salt water' becomes 'sea' (I've seen this before).
'with' means one lot of letters go next to another.
'sulphur' becomes 'S' (synonyms).
'sea'+'s'='seas'
'die' enclosing 'seas' is 'DISEASE'.
(Other definitions for disease that I've seen before include "Contagion" , "Impairment of health" , "Seaside (anag) - complaint" , "Ailment, malady" , "Illness, malady" .)