The most common and most likely answer to this crossword clue is the 4 letter word FALL
F
A
L
L
Buddy explains!

The straight part
We know that "subside" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case subside = fall.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 FALL mean?
FALL noun- The act of moving to a lower position under the effect of gravity.
- A reduction in quantity, pitch, etc.
- The time of the year when the leaves typically fall from the trees; autumn; the season of the year between the autumnal equinox and the winter solstice.
- A loss of greatness or status.
- That which falls or cascades.
- A crucial event or circumstance.
- A hairpiece for women consisting of long strands of hair on a woven backing, intended primarily to cover hair loss.
- Blame or punishment for a failure or misdeed.
- The part of the rope of a tackle to which the power is applied in hoisting (usu. plural).
- An old Scots unit of measure equal to six ells.
- A short, flexible piece of leather forming part of a bullwhip, placed between the thong and the cracker.
- The lid, on a piano, that covers the keyboard
- (heading) To be moved downwards.
- To move downwards.
- To happen, to change negatively.
- To be allotted to; to arrive through chance, fate, or inheritance.
- To diminish; to lessen or lower.
- To bring forth.
- To issue forth into life; to be brought forth; said of the young of certain animals.
- To descend in character or reputation; to become degraded; to sink into vice, error, or sin.
- To become ensnared or entrapped; to be worse off than before.
- To assume a look of shame or disappointment; to become or appear dejected; said of the face.
- To happen; to come to pass; to chance or light (upon).
- To begin with haste, ardour, or vehemence; to rush or hurry.
- To be dropped or uttered carelessly.
- (of a fabric) To hang down (under the influence of gravity).
- The chasing of a hunted whale.