The most common and most likely answer to this crossword clue is the 4 plus 3 plus 5 letter word DOWN THE HATCH
I've seen this crossword clue in Financial Times
D
O
W
N
T
H
E
H
A
T
C
H
More clues leading to the result DOWN THE HATCH
Buddy explains!
The straight part
We know that "This is said to be the precursor of a swallow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this is said to be the precursor of a swallow = down the hatch.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.