The most common and most likely answer to this crossword clue is the 5 letter word FIORD
I've seen this crossword clue in The Guardian
F
I
O
R
D
Buddy explains!

The straight part
We know that "inlet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case inlet = fiord.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 FIORD mean?
FIORD noun- A long, narrow, deep inlet between cliffs.