Following seas, moving north, swallow becomes swift to a greater extent (6,2)
I believe the answer is:
speeds up
'swift to a greater extent' is the definition.
I can't tell whether this definition defines the answer.
'following seas moving north swallow' is the wordplay.
I cannot quite understand how this works, but
'swallow' could be 'sup' ('sup' can be a synonym of 'swallow') and 'sup' is found within the answer.
The remaining letters 'speed' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'becomes' is the link.
Can you help me to learn more?
(Other definitions for speeds up that I've seen before include "Accelerates" , "Gets faster" .)