The most common and most likely answer to this crossword clue is the 4 letter word KNOT
I've seen this crossword clue in The Times
K
N
O
T
More clues leading to the result KNOT
Buddy explains!

The straight part
We know that "Bird/speed over the waves" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bird/speed over the waves = knot.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 KNOT mean?
KNOT noun- A looping of a piece of string or of any other long, flexible material that cannot be untangled without passing one or both ends of the material through its loops.
- (of hair, etc) A tangled clump.
- A maze-like pattern.
- A non-self-intersecting closed curve in (e.g., three-dimensional) space that is an abstraction of a knot (in sense 1 above).
- A difficult situation.
- The whorl left in lumber by the base of a branch growing out of the tree's trunk.
- Local swelling in a tissue area, especially skin, often due to injury.
- A protuberant joint in a plant.
- Any knob, lump, swelling, or protuberance.
- The swelling of the bulbus glandis in members of the dog family, Canidae
- The point on which the action of a story depends; the gist of a matter.
- A node.
- A kind of epaulet; a shoulder knot.
- A group of people or things.
- A bond of union; a connection; a tie.
- A unit of speed, equal to one nautical mile per hour. (From the practice of counting the number of knots in the log-line (as it is paid out) in a standard time. Traditionally spaced at one every 1/120 of a mile.)
- A nautical mile
- The bulbus glandis
- In omegaverse fiction, a bulbus glandis-like structure on the penis of a male alpha, which ties him to an omega during intercourse.
- To form into a knot; to tie with a knot or knots.
- To form wrinkles in the forehead, as a sign of concentration, concern, surprise, etc.
- To unite closely; to knit together.
- To entangle or perplex; to puzzle.
- To form knots.
- To knit knots for a fringe.
- One of a variety of shore birds; the red-breasted sandpiper (variously Calidris canutus or Tringa canutus).