Swell sky is not seen fully (5)
I believe the answer is:
heave
'swell' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for heave that I've seen before include "past Hove" , "Throw; haul by rope (naut.)" , "Produce (a sigh); retch" , "cast like this" , "Haul - retch" .)