Apprehend oceans, we're told (5)
I believe the answer is:
seize
'apprehend' is the definition.
(I know that apprehend is a more specific form of the action seize)
'oceans we're told' is the wordplay.
'oceans' becomes 'seas' ('sea' can be a synonym of 'ocean').
'we're told' shows a homophone (sound like).
'seas' is a homophone of 'SEIZE'.
(Other definitions for seize that I've seen before include "Take hold of, grab" , "Snatch" , "Take (an opportunity)" , "Capture" , "Take by force" .)