Completely fill it at sea when stormy (7)
I believe the answer is:
satiate
'completely fill' is the definition.
'satiate' can be an answer for 'fill' ('satiate' can be a synonym of 'fill'). I'm unsure of the 'completely' bit.
'it at sea when stormy' is the wordplay.
'when stormy' indicates anagramming the letters (I've seen this in other clues).
'it'+'at'+'sea'='itatsea'
'itatsea' anagrammed gives 'SATIATE'.
Can you help me to learn more?
(Other definitions for satiate that I've seen before include "Have more than enough" , "Satisfy fully, cloy with excess" , "Completely satisfy" , "stuff" , "Overfill - gorge" .)