Seas to rise and move rapidly (5)
I believe the answer is:
speed
'move rapidly' is the definition.
'seas to rise' is the wordplay.
'seas' becomes 'deeps' (the deep can mean the sea).
'to rise' says the letters should be written backwards.
'deeps' in reverse letter order is 'SPEED'.
'and' acts as a link.
(Other definitions for speed that I've seen before include "Film sensitivity to light" , "Rapidity of movement" , "Hasten" , "Pace or tempo" , "A devilish quick driver?" .)