Perish going across oceans — could this be the cause? (7)
I believe the answer is:
disease
'this be the cause?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'perish going across oceans' is the wordplay.
'perish' becomes 'die' (synonyms).
'going across' indicates putting letters inside.
'oceans' becomes 'seas' ('sea' can be a synonym of 'ocean').
'die' going around 'seas' is 'DISEASE'.
'could' acts as a link.
(Other definitions for disease that I've seen before include "affection" , "Ailment, malady" , "Seaside (anag) - complaint" , "Seaside (anag) -- ailment" , "blight" .)