The most common and most likely answer to this crossword clue is the 4 letter word BALK
B
A
L
K
More clues leading to the result BALK
Buddy explains!

The straight part
We know that "Decline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decline = balk.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 BALK mean?
BALK noun- An uncultivated ridge formed in the open field system, caused by the action of ploughing.
- The wall of earth at the edge of an excavation.
- Beam, crossbeam; squared timber; a tie beam of a house, stretching from wall to wall, especially when laid so as to form a loft, "the balks".
- A hindrance or disappointment; a check.
- A sudden and obstinate stop; a failure.
- An omission.
- A deceptive motion; a feint.
- The area of the table lying behind the line from which the cue ball is initially shot, and from which a ball in hand must be played.
- The area of the table lying behind the baulk line.
- The rope by which fishing nets are fastened together.
- To pass over or by.
- To omit, miss or overlook by chance.
- To miss intentionally; to avoid.
- To stop, check, block.
- To stop short and refuse to go on.
- To refuse suddenly.
- To disappoint; to frustrate.
- To engage in contradiction; to be in opposition.
- To leave or make balks in.
- To leave heaped up; to heap up in piles.
- To indicate to fishermen, by shouts or signals from shore, the direction taken by the shoals of herring.