The most common and most likely answer to this crossword clue is the 4 letter word WAVE
I've seen this crossword clue in Independent
W
A
V
E
More clues leading to the result WAVE
- Fail to apply for audition, and flourish
- Gesture with a heartless vote
- See 11ac.
- What toppled surfer with a heartless violence?
- Swell way to say goodbye
- Surge of water with hail
- Signal breaker, for example
- Greeting couples midway through Newark tavern
- Bridge partners holding ace and five, producing movement by Queen, perhaps?
- Marine feature which may be permanent
Buddy explains!

The straight part
We know that "Swell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swell = wave.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 WAVE mean?
WAVE verb- To move back and forth repeatedly and somewhat loosely.
- To move one’s hand back and forth (generally above the shoulders) in greeting or departure.
- (metonymic) To call attention to, or give a direction or command to, by a waving motion, as of the hand; to signify by waving; to beckon; to signal; to indicate.
- To have an undulating or wavy form.
- To raise into inequalities of surface; to give an undulating form or surface to.
- To produce waves to the hair.
- To swing and miss at a pitch.
- To cause to move back and forth repeatedly.
- (metonymic) To signal (someone or something) with a waving movement.
- To fluctuate; to waver; to be in an unsettled state.
- To move like a wave, or by floating; to waft.
- A moving disturbance in the level of a body of liquid; an undulation.
- The ocean.
- A moving disturbance in the energy level of a field.
- A shape that alternatingly curves in opposite directions.
- Any of a number of species of moths in the geometrid subfamily Sterrhinae, which have wavy markings on the wings.
- A loose back-and-forth movement, as of the hands.
- A sudden unusually large amount of something that is temporarily experienced.
- (by extension) One of the successive swarms of enemies sent to attack the player in certain games.
- (usually "the wave") A group activity in a crowd imitating a wave going through water, where people in successive parts of the crowd stand and stretch upward, then sit.
- To relinquish (a right etc.); to give up claim to; to forego.
- To put aside, avoid.
- To outlaw (someone).
- To abandon, give up (someone or something).
- To move from side to side; to sway.
- To stray, wander.