The most common and most likely answer to this crossword clue is the 7 letter word PROTEUS
I've seen this crossword clue in The Times
P
R
O
T
E
U
S
Buddy explains!

The straight part
We know that "Sea god" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sea god = proteus.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PROTEUS mean?
PROTEUS noun- Any of many gram-negative bacteria, of the genus Proteus, several of which are responsible for human infections.