The most common and most likely answer to this crossword clue is the 4 letter word BENT
I've seen this crossword clue in Independent
B
E
N
T
More clues leading to the result BENT
Buddy explains!

The straight part
We know that "Natural aptitude/on the fiddle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case natural aptitude/on the fiddle = bent.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 BENT mean?
BENT verb- To cause (something) to change its shape into a curve, by physical force, chemical action, or any other means.
- To become curved.
- To cause to change direction.
- To change direction.
- To be inclined; to direct itself.
- (usually with "down") To stoop.
- To bow in prayer, or in token of submission.
- To force to submit.
- To submit.
- To apply to a task or purpose.
- To apply oneself to a task or purpose.
- To adapt or interpret to for a purpose or beneficiary.
- To tie, as in securing a line to a cleat; to shackle a chain to an anchor; make fast.
- To smoothly change the pitch of a note.
- To swing the body when rowing.
- An inclination or talent.
- A predisposition to act or react in a particular way.
- The state of being curved, crooked, or inclined from a straight line; flexure; curvity.
- A declivity or slope, as of a hill.
- Particular direction or tendency; flexion; course.
- A transverse frame of a framed structure; a subunit of framing.
- Tension; force of acting; energy; impetus.
- (Of something that is usually straight) folded, dented
- Corrupt, dishonest
- Homosexual.
- Determined or insistent.
- (Of a person) leading a life of crime.
- Inaccurately aimed
- Suffering from the bends
- High from both marijuana and alcohol.
- Any of various stiff or reedy grasses.
- A grassy area, grassland.
- The old dried stalks of grasses.