Illness causing to perish going round the oceans (7)
I believe the answer is:
disease
'illness' is the definition.
(I know that illness can be written as disease)
'to perish going round the oceans' is the wordplay.
'to perish' becomes 'die' ('die' can be a synonym of 'perish').
'going round' is an insertion indicator (some letters go around others).
'the oceans' becomes 'seas' ('sea' can be a synonym of 'ocean').
'die' enclosing 'seas' is 'DISEASE'.
'causing' is the link.
(Other definitions for disease that I've seen before include "Impairment of health" , "scrapie for example" , "Ailment, malady" , "Illness, malady" , "22 [MALADY]" .)