Be afraid of the sea monster, ultimately (4)
I believe the answer is:
fear
'be afraid' is the definition.
(I've seen this before)
'of the sea monster ultimately' is the wordplay.
'ultimately' says to take the final letters.
The last letters of 'of the sea monster' are 'FEAR'.
(Other definitions for fear that I've seen before include "Dread, alarm" , "awe" , "Alarm, reverence" , "Terror" , "Be afraid of" .)