Illness having one fade away when crossing oceans? (7)
I believe the answer is:
disease
'illness' is the definition.
(I know that illness can be written as disease)
'one fade away when crossing oceans?' is the wordplay.
'one fade away' becomes 'die' (I've seen this in other clues).
'when crossing' indicates putting letters inside.
'oceans?' becomes 'seas' ('sea' can be a synonym of 'ocean').
'die' enclosing 'seas' is 'DISEASE'.
'having' acts as a link.
(Other definitions for disease that I've seen before include "Contagion" , "Seaside (anag) -- ailment" , "scrapie for example" , "Ailment, malady" , "Sickness" .)